import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        char[] s = sc.next().toCharArray();
        int n = s.length;
        int max = 0;
        for (int i = 0 ; i < n ; i++) {
            for (int j = i ; j < n ; j++) {
                boolean flg = true;
                for (int k = i ; k <= j ; k++) {
                    if (s[k] != 'A' && s[k] != 'T' && s[k] != 'C' && s[k] != 'G') flg = false;
                }
                if (flg) max = Math.max(max, j - i + 1);
            }
        }
        System.out.println(max);

    }

}

/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