import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner s = new Scanner(System.in);
    String S = s.nextLine();
    int count = 0,ans = 0;

    for(int i = 0; i < S.length(); i++){
      if(S.charAt(i) == 'A' || S.charAt(i) == 'C' || S.charAt(i) == 'G' || S.charAt(i) == 'T'){
        count++;
        if(ans < count){
          ans = count;
        }
      }
      else{
        count = 0;
      }
    }
    System.out.println(ans);
  }
}

/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