import java.util.*;
public class Main {

  public static void main(String[] args) { 
      Scanner sc = new Scanner(System.in);
      int a = sc.nextInt();
      int b = sc.nextInt();
      int x = 1;
      while(true) {
          int A = (int)(x * 0.08 * 10) / 10;
          int B = (int)(x * 0.1 * 10) / 10;
        if(A == a && B == b) {
            System.out.println(x);
            return;
        }
        if(A > a && B > b)
            break;
        x++;
      }
      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