import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();

        for (int i = 0; i < Integer.MAX_VALUE; i++) {
            int tax8 = (int) (i * 0.08);
            int tax10 = (int) (i * 0.1);
            if (a == tax8 && b == tax10) {
                System.out.println(i);
                return;
            }
            if (a < tax8 || b < tax10) {
                System.out.println("-1");
                return;
            }
        }
    }
}

/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