import java.util.Scanner;
import java.lang.Math;
public class Main {
    public static void main (String...args) {
        Scanner sc = new Scanner(System.in); int a = sc.nextInt(); int b = sc.nextInt(); int answer = -1;
        for (int i=b*10; i<(b+1)*10; i++) if (i*8/100==a){ answer = i; break;}
        System.out.println(answer);}
}
/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