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;
		int b = 0;

		char[] c = S.toCharArray();
		for (int i = 0; i < S.length(); i++) {
			if (c[i] == 'A' || c[i] == 'C' || c[i] == 'G' || c[i] == 'T') {
				a++;
			} else {
				b = 1;
				if (max < a) {
					max = a;
				}
				a = 0;
			}
			if (max < a) {
				max = a;
			}
		}
		if (b == 0) {
			System.out.println(S.length());
		} else {
			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