import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		char[] s = sc.next().toCharArray();
		int count = 0; boolean flag = false; int max = 0;
		for (int i = 0; i < s.length; i++) {
			if(flag) {
				if(s[i]=='A'||s[i]=='T'||s[i]=='G'||s[i]=='C') count++;
				else {
					flag = false;
					max = Math.max(max,count);
					count = 0;
				}
			}else {
				if(s[i]=='A'||s[i]=='T'||s[i]=='G'||s[i]=='C') {
					flag = true;
					count = 1;
				}
			}
		}
		max = Math.max(max,count);
		System.out.println(max);
		sc.close();
	}
}
/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