import java.util.Scanner;

public class Main {

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