import java.util.*;
public class Main{
    public static void main(String [] args){
        Scanner sc= new Scanner(System.in);
        String s= sc.nextLine();
        int count=0;
        int max=0;
        for(int i=0; i<s.length(); i++){
            char c= s.charAt(i);
            if(c=='A'||c=='C'||c=='G'||c=='T') count++;
            else count=0;
            
            if(count>max) max=count;
        }
        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