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();
        for(double i=1;i<10000;i+=1){
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)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);
        int a=sc.nextInt();
        int b=sc.nextInt();
        for(double i=1;i<10000;i+=1){
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)i);
                return;
            }
        }
        System.out.println(-1);
        
        
 
    }



} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        for(double i=1;i<10000;i+=1){
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)i);
                return;
            }
        }
        System.out.println(-1);
        
        
 
    } main {
        Scanner sc = new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        for(double i=1;i<10000;i+=1){
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)i);
                return;
            }
        }
        System.out.println(-1);
        
        
 
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a=sc.nextInt(); a sc.nextInt() sc.nextInt sc int b=sc.nextInt(); b sc.nextInt() sc.nextInt sc for(double i=1;i<10000;i+=1){
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)i);
                return;
            }
        } double i=1; double i=1; i 1 i<10000 i 10000 i+=1 i+=1 i 1 {
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)i);
                return;
            }
        } {
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)i);
                return;
            }
        } if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)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((int)i);
                return;
            } System.out.println((int)i) System.out.println System.out System System.out (int)i return; System.out.println(-1) System.out.println System.out System System.out -1 1 String[] args args public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        for(double i=1;i<10000;i+=1){
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)i);
                return;
            }
        }
        System.out.println(-1);
        
        
 
    }



} public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        for(double i=1;i<10000;i+=1){
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)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);
        int a=sc.nextInt();
        int b=sc.nextInt();
        for(double i=1;i<10000;i+=1){
            if((int)(i*0.08)==a&&(int)(i*0.1)==b){
                System.out.println((int)i);
                return;
            }
        }
        System.out.println(-1);
        
        
 
    }



}