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 count = 0;
		for(int i = 0; i <= S.length(); i++) {
			int tmpCount = 0;
			for(int j = i + 1; j <= S.length(); j++) {
				String tmp = S.substring(i, j);
				int num = tmp.length() - 1;
				if(tmp.substring(num).equals("A") || tmp.substring(num).equals("C") || tmp.substring(num).equals("G") || tmp.substring(num).equals("T")) {
					tmpCount++;
				}else {
					break;
				}
			}
			if(count < tmpCount) count = tmpCount;
		}

		System.out.println(count);
	}
}
/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