import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		int A;
		int B;
		int i;
		Scanner sc = new Scanner(System.in);
		A = sc.nextInt();
		B = sc.nextInt();
		sc.close();
		for (i = 0; i < 10001; i++) {
			if ((int) (i * 0.08) == A && (int) (i * 0.1) == B) {
				System.out.println(i);
				return;
			}
		}
		System.out.println("-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