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 count = 0;
    int ans = 0;
    for (int i = 0; i < n; i++) {
      if (s.charAt(i) == 'A' || s.charAt(i) == 'C' || s.charAt(i) == 'G' || s.charAt(i) == 'T') {
        count++;
      } else {
        ans = Math.max(ans, count);
        count = 0;
      }
    }
    ans = Math.max(ans, count);
    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