import java.util.Scanner;

public class Main {
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    String s=sc.next();
    String[] ss =s.split("",0);
    int max=0,index=0;
    for (String value : ss) {
      if (value.equals("A") || value.equals("C") || value.equals("G") || value.equals("T")) {
        index++;
        max = Math.max(max, index);
      } else {
        max = Math.max(max, index);
        index = 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