import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        // ABC158C - Tax Increase

        Scanner sc = new Scanner(System.in);

        int a = sc.nextInt();
        int b = sc.nextInt();

        int ans = -1;
        for (int i = 1; i <= 10000; i++) {
            if ((int)(i * 0.08) == a && 
                (int)(i * 0.10) == b) {
                    ans = i;
                    break;
                }
        }
        
        
       System.out.println(ans);
    }
}
/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