import java.util.*;

public class Main {

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

        int ans = 0;
        int count = 0;
        for (Character c : s.toCharArray()) {
            if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
                count++;
            } else {
                if (ans < count) {
                    ans = count;
                    count = 0;
                }
            }
        }

        System.out.print(Math.max(ans, count));
    }

}
/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