import java.util.*;
import java.lang.*;
import java.io.*;


public class Main {
    public static void main(String[] args) throws java.lang.Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader((System.in)));
        StringTokenizer s = new StringTokenizer(br.readLine());
        int a = Integer.parseInt(s.nextToken());
        int b = Integer.parseInt(s.nextToken());
        int yen = b * 10;
        while (yen < (b+1) * 10 && (int)(yen * 0.08) != a)
            yen++;
        System.out.println(yen < (b+1)*10 ? yen : -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