import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
        //int N = sc.nextInt();
        char[] S = sc.next().toCharArray();
        int count=0;
        int max = 0;
        for(int i=0; i<S.length; i++){
            if(S[i]=='A' || S[i]=='G' || S[i]=='C' || S[i]=='T'){
                  count++;
                  max = Math.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