import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    String S = sc.next();
    int l=0;
    int ret=0;
    for(char c: S.toCharArray()){
      switch(c){
        case 'A':
        case 'T':
        case 'C':
        case 'G':
          l++; break;
        default:
          l=0;
      }
      ret = Math.max(ret, l);
    }
    System.out.println(ret);
  }
}
/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