import java.util.Scanner;

public class Main {

  public static void main(String[] args) {

    Scanner scan = new Scanner(System.in);

    String str = scan.next();
    int max = 0;

    for (int i = 0; i < str.length();) {
      int cnt = 0;
      for (int j = i; j < str.length(); j++) {
        i = j + 1;
        if ((str.charAt(j) == 'A') || (str.charAt(j) == 'C') || (str.charAt(j) == 'G') || (str.charAt(j) == 'T')) {
          cnt++;
        } else {
          break;
        }
      }
      max = Math.max(max, cnt);
    }

    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