import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        char[] s = sc.next().toCharArray();
        int ans = 0;
        for (int i = 0; i < s.length; i++) {
            if (s[i] == 'A' || s[i] == 'C' || s[i] == 'G' || s[i] == 'T') {
                int temp = 0;
                for (int j = i; j < s.length; j++) {
                    if (s[j] == 'A' || s[j] == 'C' || s[j] == 'G' || s[j] == 'T') {
                        temp += 1;
                    } else {
                        break;
                    }
                } ans = Math.max(ans, temp);
            }
        } System.out.println(ans);
    }
}

/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