import java.io.*;

public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    
    char[] t = br.readLine().toCharArray();
    
    int result = 0;
    int temp = 0;
    for (char c : t){
      if (c == 'A' || c == 'C' || c == 'G' || c == 'T') {
        temp++;
        if (result < temp) result = temp;
      } else {
        temp = 0;
      }
    }
    System.out.println(result);
  }
}
/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