import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		String s=sc.nextLine();
		int cnt=0, ans=0;
		for(char c: s.toCharArray()) {
			if(c=='A' || c=='C' || c=='G' || c=='T')
				cnt++;
			else
				cnt=0;
			ans=Math.max(cnt, ans);
		}
		System.out.println(ans);
	}
}
/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