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();
		sc.close();

		int max = (int) (100d / 0.08d);

		for (int i = 1; i <= max; i++) {
			int xa = (int) (i * 0.08D);
			int xb = (int) (i * 0.1D);
			if (a == xa && b == xb) {
				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