import java.util.*;


public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();

        int current = 0;
        int max = 0;
        for(int i = 0 ; i<s.length(); i++){
            char x = s.charAt(i);
            if(x=='A' || x=='C'|| x=='G'||x=='T'){
                current++;
            }else{
                current = 0;
            }
            max = Math.max(current,max);
        }
        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