import java.util.*;
import java.math.*;
public class Main {
	static int r; 
	static int c;
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		int a = in.nextInt();
		int b = in.nextInt();
		boolean[] ans = new boolean[1001];
		for(int i = 1; i < 1001; i++) {
			int eight = (int)(i * 0.08);
			int ten = (int) (i * 0.1);
			ans[i] = ((eight == a) && (ten == b));
		}
		String res = "-1";
		for(int i = 1; i < 1001; i++) {
			if(ans[i] == true) {
				res = i + "";
				break;
			}
		}
		System.out.println(res);
	}
} 

/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