import java.util.*;
import java.math.*;

public class Main{
	public static void main(String[] args) {
        Scanner itr = new Scanner(System.in);
        int A = itr.nextInt();
        int B = itr.nextInt();

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