
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.NoSuchElementException;

/**
 * Created by jaga on 9/22/16.
 */
public class Main {
    public static void solve(PrintWriter out, FastScanner in){
        String a = in.next();
        String b = in.next();
        String c = in.next();

        int na = a.length();
        int nb = b.length();
        int nc = c.length();

        int ia = 1;
        int ib = 0;
        int ic = 0;

        char prev = a.charAt(0);

        loop:while (ia <= na   && ib <= nb  && ic <= nc ) {
            switch (prev){
                case 'a':
                    if (ia == na) {
                        System.out.println("A");
                        break loop;
                    }
                    prev = a.charAt(ia);
                    ia ++;
                    break;
                case 'b':
                    if (ib == nb) {
                        System.out.println("B");
                        break loop;
                    }
                    prev = b.charAt(ib);
                    ib++;
                    break;
                case 'c':
                    if (ic == nc) {
                        System.out.println("C");
                        break loop;
                    }
                    prev = c.charAt(ic);
                    ic ++;
                    break;
            }
        }
    }

    static class Pair implements Comparable {
        int first, second;

        Pair(int a, int b) {
            first = a;
            second = b;
        }

        public int compareTo(Object other) {
            Pair p1 = (Pair) other;
            return this.first - ((Pair) other).first; // IDの値に従い昇順で並び替えたい場合
//         return -(this.first - ((Pair) other).first); // IDの値に従い降順で並び替えたい場合
        }
    }

    /*以下はテンプレ*/
    public static void main(String args[]) {
        OutputStream outputStream = System.out;
        PrintWriter out = new PrintWriter(outputStream);
        FastScanner in = new FastScanner();
        solve(out, in);
        out.close();
    }

    public static class FastScanner {
        private final InputStream in = System.in;
        private final byte[] buffer = new byte[1024];
        private int ptr = 0;
        private int bufferLength = 0;

        private boolean hasNextByte() {
            if (ptr < bufferLength) {
                return true;
            } else {
                ptr = 0;
                try {
                    bufferLength = in.read(buffer);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                if (bufferLength <= 0) {
                    return false;
                }
            }
            return true;
        }

        private int readByte() {
            if (hasNextByte()) return buffer[ptr++];
            else return -1;
        }

        private static boolean isPrintableChar(int c) {
            return 33 <= c && c <= 126;
        }

        private void skipUnprintable() {
            while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        }

        boolean hasNext() {
            skipUnprintable();
            return hasNextByte();
        }

        public String next() {
            if (!hasNext()) throw new NoSuchElementException();
            StringBuilder sb = new StringBuilder();
            int b = readByte();
            while (isPrintableChar(b)) {
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }

        long nextLong() {
            if (!hasNext()) throw new NoSuchElementException();
            long n = 0;
            boolean minus = false;
            int b = readByte();
            if (b == '-') {
                minus = true;
                b = readByte();
            }
            if (b < '0' || '9' < b) {
                throw new NumberFormatException();
            }
            while (true) {
                if ('0' <= b && b <= '9') {
                    n *= 10;
                    n += b - '0';
                } else if (b == -1 || !isPrintableChar(b)) {
                    return minus ? -n : n;
                } else {
                    throw new NumberFormatException();
                }
                b = readByte();
            }
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        double[] nextDoubleArray(int n) {
            double[] array = new double[n];
            for (int i = 0; i < n; i++) {
                array[i] = nextDouble();
            }
            return array;
        }

        double[][] nextDoubleMap(int n, int m) {
            double[][] map = new double[n][];
            for (int i = 0; i < n; i++) {
                map[i] = nextDoubleArray(m);
            }
            return map;
        }

        public int nextInt() {
            return (int) nextLong();
        }

        public int[] nextIntArray(int n) {
            int[] array = new int[n];
            for (int i = 0; i < n; i++) array[i] = nextInt();
            return array;
        }

        public long[] nextLongArray(int n) {
            long[] array = new long[n];
            for (int i = 0; i < n; i++) array[i] = nextLong();
            return array;
        }

        public String[] nextStringArray(int n) {
            String[] array = new String[n];
            for (int i = 0; i < n; i++) array[i] = next();
            return array;
        }

        public char[][] nextCharMap(int n) {
            char[][] array = new char[n][];
            for (int i = 0; i < n; i++) array[i] = next().toCharArray();
            return array;
        }

        public int[][] nextIntMap(int n, int m) {
            int[][] map = new int[n][];
            for (int i = 0; i < n; i++) {
                map[i] = nextIntArray(m);
            }
            return map;
        }
    }
}


/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz