import java.util.Scanner;

public class Main {
	public static void main(final String[] args) {
		Scanner sc = new Scanner(System.in);
		char[] S = sc.next().toCharArray();
		sc.close();
		int max = 0;
		int cnt = 0;
		for(int i = 0; i < S.length; i++) {
			if(S[i] == 'A' || S[i] == 'C' || S[i] == 'G' || S[i] == 'T') {
				cnt++;
			} else {
				max = Math.max(max, cnt);
				cnt = 0;
			}
		}
		max = Math.max(max, cnt);
		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