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();

		int ans = -1;
		for (int i=0; i<10; i++) {
			int x = B*10 + i;
			if (x * 8 / 100 == A) {
				ans = x;
				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