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();

		boolean bool = true;
		loop: for (int i = 1; i <= 1010; i++) {
			int k1 = (int) (i * 0.08);
			int k2 = (int) (i * 0.1);

			if (k1 == a && k2 == b) {
				bool = false;
				System.out.println(i);
				break loop;
			}
		}

		if (bool) {
			System.out.println("-1");
		}

		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