import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int a = scan.nextInt();
    int b = scan.nextInt();

    long c = b*10;
    for (long j=c;j<c+10;j++){
      double gg = j*0.08;
      String t = gg+"";
      String p = a+"";
      boolean found = false;
      for (int l = 0; l<p.length();l++){
        if (t.charAt(l) == p.charAt(l)){
          found = true;
        }
        else{
          found = false;
          break;
        }
      }
      if (found == true){
        System.out.println(j);
        return;
      }
    }
    System.out.println(-1);
  }
}

/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