import java.util.*;

public class Main {
	public static void main(String[] args) throws Exception{
		Scanner scn = new Scanner(System.in);
		String s = scn.next();
		int max = 0;
		int count = 0;
		for(int i=0; i<s.length(); i++){
			if(s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T'){
				count++;
				if(count > max){
					max = count;
				}
			}else{
				count = 0;
			}
		}
		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