import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;
public class Main {
    public static void main(String[] args){
    
    Scanner sc = new Scanner(System.in);
        String S = sc.next();
        char[]ss = S.toCharArray();
        ArrayList<Integer>ans  = new ArrayList<>();
        
        int cou = 0;
        for(int i=0; i<S.length(); i++) {
        	if(ss[i]=='A' || ss[i]=='C' || ss[i]=='G' || ss[i]=='T'){
        		cou++;
        	}else {
        		ans.add(cou);
        		cou = 0;
        	}
        }
        ans.add(cou);
        Collections.sort(ans,Collections.reverseOrder());	
            System.out.println(ans.get(0));
    }
}
/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