import java.util.*;
public class Main {
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int max = 0,count=0,len=s.length();
    for(int i = 0;i < len;i++){
      switch(s.charAt(i)){
        case 'A':
          count++;
          break;
        case 'T':
          count++;
          break;
        case 'G':
          count++;
          break;
        case 'C':
          count++;
          break;
        default:
          count=0;
      }
      if(max < count)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