import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s =sc.next();
    System.out.println(solve(s));
  }

  private static int solve(String s) {
    int ret = 0;
    for(int i=0; i<s.length(); i++) {
      for(int j=i+1; j<=s.length(); j++) {
        ret = Math.max(ret, solve2(s.substring(i, j)));
      }
    }
    return ret;
  }
  
  private static int solve2(String s) {
    for(char c: s.toCharArray()) {
      if(c!='A' && c!='C' && c!='G' && c!='T') return 0;
    }
    return s.length();
  }
}

/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