import java.util.Scanner;


public class Main {


	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ


		Scanner sc = new Scanner(System.in);
		String S = sc.next();
		
		int count = 0;
		int res = 0;
		
		for(int i =0;i<S.length();i++) {
			char c = S.charAt(i);
			if(c=='A'||c=='G'||c=='C'||c=='T') {
				count++;
			}else {
				res = Math.max(res, count);
				count=0;
			}
		}
		
		res = Math.max(res, count);
		System.out.println(res);


		}


}
/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