import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
   
    for(int i=0;i<=1000;i++){
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    }
    System.out.println("-1");
  }
} import java.util.*; util.* java public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
   
    for(int i=0;i<=1000;i++){
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    }
    System.out.println("-1");
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
   
    for(int i=0;i<=1000;i++){
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    }
    System.out.println("-1");
  } main {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
   
    for(int i=0;i<=1000;i++){
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    }
    System.out.println("-1");
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long a = sc.nextLong(); a sc.nextLong() sc.nextLong sc long b = sc.nextLong(); b sc.nextLong() sc.nextLong sc for(int i=0;i<=1000;i++){
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    } int i=0; int i=0; i 0 i<=1000 i 1000 i++ i++ i {
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    } {
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    } if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      } (int)(i*0.08)==a&&(int)(i*0.1)==b (int)(i*0.08)==a (int)(i*0.08) i 0.08 a (int)(i*0.1)==b (int)(i*0.1) i 0.1 b {
        System.out.println(i);
        return;
      } System.out.println(i) System.out.println System.out System System.out i return; System.out.println("-1") System.out.println System.out System System.out "-1" String[] args args public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
   
    for(int i=0;i<=1000;i++){
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    }
    System.out.println("-1");
  }
} public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
   
    for(int i=0;i<=1000;i++){
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    }
    System.out.println("-1");
  }
} Main
import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
   
    for(int i=0;i<=1000;i++){
      if((int)(i*0.08)==a&&(int)(i*0.1)==b){
        System.out.println(i);
        return;
      }
    }
    System.out.println("-1");
  }
}