import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int max = 0;

		String s = sc.next();
		for(int i = 0; i < s.length(); i++) {
			for(int j = s.length(); j >i;j--) {
				String temp = s.substring(i,j);
				if(temp.matches("[A,T,C,G]{0,}")) {
					
				if(temp.length() > max) {
					max = temp.length();
				}
				}
			}
		}
		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