import java.io.*;
import java.util.*;
import java.math.*;
     
public class Main{
	public static void main(String[] args) throws Exception {
	IO io = new IO();
	PrintWriter out = new PrintWriter(System.out);
	Solver sr = new Solver();
	sr.solve(io,out);
	out.flush();
	out.close();
    	}

	static class Solver
	{
	    void solve(IO io, PrintWriter out)
	    {
          int x = io.nextInt();
          int y = io.nextInt();
          int min=-1;
          for(int i=1 ; i<=1250 ; i++)
          {
            int tmp = (int)Math.floor((i*8)/100);
            int tmp1 = (int)Math.floor(i/10);
            if(tmp==x && tmp1==y){
              min=i;
              break;
            }
          }
          out.print(min);
	    }
    	
	}
	//Special thanks to Petr (on codeforces) who inspired me to implement IO class!
	static class IO
	{
    	BufferedReader reader;
        StringTokenizer tokenizer;
    	public IO() {
                reader = new BufferedReader(new InputStreamReader(System.in));
                tokenizer = null;
            }
     
        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }
        public String nextLine() {
            String s="";
            try {
                    s=reader.readLine();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            return s;
        }
        public int nextInt() {
            return Integer.parseInt(next());
        }
    
        public long nextLong() {
            return Long.parseLong(next());
        }
    	double nextDouble()
    	{
    		return Double.parseDouble(next());
    	}
	}
}
/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