import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int n = s.length();
    int max = 0;
    int now = 0;
    for(int i = 0;i<n;i++){
      now = 0;
      for(int j = i + 1;j<=n;j++){
        String ss = s.substring(j-1,j);
        if(ss.equals("A") || ss.equals("C") || ss.equals("G") || ss.equals("T")){
          now++;
          continue;
        }else{
          break;
        }
      }
      if(max < now){
        max = now;
      }
    }
    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