import java.util.Scanner;

class Solver {
    public void Solve() {
        int a = sc.nextInt();
        int b = sc.nextInt();
        for (int i = 1; i < 1100; i++) {
            if (Math.floor(i * 0.08) == a && Math.floor(i * 0.1) == b) {
                System.out.println(i);
                return;
            }
        }
        System.out.println(-1);
    }

    Scanner sc;

    Solver(Scanner in) {
        this.sc = in;
    }
}

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        Solver s = new Solver(sc);
        s.Solve();
        sc.close();
    }
}

/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