import java.util.*;

public class Main {
    public static void main(String args[] ) throws Exception {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        Set<Character> letters = new HashSet<>();
        letters.add('A');
        letters.add('T');
        letters.add('C');
        letters.add('G');

        int count = 0;
        int max = 0;

        for (char c : s.toCharArray()) {
            if (letters.contains(c)) count++;
            else count = 0;
            max = Math.max(max, count);
        }
        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