import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();

        int max = 0;
        int tmp = 0;
        for (int i = 0; i < s.length(); ++i) {
            char ch = s.charAt(i);
            if (ch == 'A' || ch == 'C' || ch == 'G' || ch == 'T') ++tmp;
            else tmp = 0;
            if (tmp > max) max = tmp;
        }

        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