import java.util.*;

public class Main {
	void solve() {
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		for(int i = 1; i <= 100000; i++) {
			if((int)(i * 0.08) == a && (int)(i * 0.1) == b) {
				System.out.println(i);
				return;
			}
		}
		System.out.println(-1);
		scan.close();
	}
	
	public static void main(String[] args) {
		new Main().solve();
	}
}

/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