import java.io.*;
import java.util.*;

class Main {

	public static void main(String[] args) throws Exception {
		final Scanner sc = new Scanner(System.in);
		String s = sc.next();
		int a = 0;
		Set<Character> valid = new HashSet<>(Arrays.asList('A', 'G', 'C', 'T'));
		for (int i = 0; i < s.length(); i++) {
			int count = 0;
			int j = i;
			while (j < s.length() && valid.contains(s.charAt(j++))) count++;
			a = Math.max(a, count);
		}

		System.out.println(a);
	}
}

/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