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 ans = 0;
    for (int i = 0; i < n; i++) {
      for (int j = i + 1; j <= n; j++) {
        String str = s.substring(i, j);
        int cnt = 0;
        for (int k = 0; k < str.length(); k++) {
          char ch = str.charAt(k);
          if (!(ch == 'A' || ch == 'C' || ch == 'G' || ch == 'T')) {
            cnt = 0;
            break;
          }
          cnt++;
        }
        ans = Math.max(ans, cnt);
      }
    }
    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