////////////////////////////////////////////////////
    //                                                //
    //  For her who keeps the fire kindling in me...  //
    //                                                //
    ////////////////////////////////////////////////////


    import java.io.BufferedReader;
    import java.io.BufferedWriter;
    import java.io.FileOutputStream;
    import java.io.IOException; 
    import java.io.InputStreamReader;
    import java.io.OutputStream;
    import java.io.OutputStreamWriter;
    import java.io.PrintWriter;
    // import java.util.Scanner;
    // import java.util.Arrays;
    // import java.util.ArrayList;
    // import java.util.Collections; 
    // import java.util.StringTokenizer;
    import java.util.*;
    import java.lang.Math;
    import java.awt.Point;                            
    
    import java.awt.geom.*;
    
    public class Main{
    
        static class Pair{
            int x;
            int y;
        }
    
        public static void main(String args[]){
            try{
                FastReader s = new FastReader();
                BufferedWriter w = new BufferedWriter(new OutputStreamWriter(System.out));
                int i, j, k, x, y, t, m, l, fo, so, ev, r, count, key, size, Pairer, minglob, minloc, beg, ec, oc, end, begl, endr, ld, up, down, maxele, minele;
                long ans, sqrt, q, sum, maxup, maxdown, X, Y, Q, MOD, n, a, b, p1, p2, p, ac, bc;//, sum2;
                boolean flag, flag2;
    
                double avg;
    
                    // t = s.nextInt();
    
                    //for(; t>0; t--){
    
                    // n = s.nextInt();
    
                    // int arr[] = new int[n];
    
                    // flag = false;   flag2 = false;
                    // ev = -1;    fo = -1; sp = -1; ec=0; oc = 0;
                    // for(i=0; i<n; i++){
                    //     x = s.nextInt();
                    //     if(x%2==0  && ev != -1){
                    //         ev = x;
                    //         ec++;
                    //     }else if(x%2 == 1){
                    //         if(fo == -1)
                    //             fp = x;
                    //         else
                    //             so = x;
    
                    //         oc++;
                    //     }
                    // }o
    
                    //n = s.nextLong();
                    a = s.nextLong();
                    b = s.nextLong();
    
    
                    p1 = (long)Math.ceil((a*100)/8f);
                    p2 = (long)Math.ceil((b*100)/10f);
                    p = Math.max(p1,  p2);
                
                    ac = (p*8)/100;
                    bc = (p*10)/100;
    
                    if(ac == a  &&  bc==b)
                        w.write(p+"\n");
    
                    else
                    
                        w.write("-1\n");
    
                    
    
                    
                
                    //}
                w.close();
            }catch(Exception e){
                e.printStackTrace();
            }
        }
    }
                    
                    
    //========================================### FAST IO ###=========================================//
    class FastReader 
    { 
        BufferedReader br; 
        StringTokenizer st; 
    
        public FastReader() 
        { 
            br = new BufferedReader(new
                        InputStreamReader(System.in)); 
        } 
    
        String next() 
        { 
            while (st == null || !st.hasMoreElements()) 
            { 
                try
                { 
                    st = new StringTokenizer(br.readLine()); 
                } 
                catch (IOException  e) 
                { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
    
        int nextInt() 
        { 
            return Integer.parseInt(next()); 
        } 
    
        long nextLong() 
        { 
            return Long.parseLong(next()); 
        } 
    
        Double nextDouble() 
        { 
            return Double.parseDouble(next());
        } 
    
        String nextLine() 
        { 
            String str = ""; 
            try
            { 
                str = br.readLine(); 
            } 
            catch (IOException e) 
            { 
                e.printStackTrace(); 
            } 
            return str;
        } 
    }
        //====================================================================================================//

Measured memory_bytes: 22484