import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here! 
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        for (int i = 1; i <= 1001001009; i++) {
            double tmp1 = i*0.08;
            int tmpa = (int)tmp1;
            double tmp2 = i*0.1;
            int tmpb = (int)tmp2;
            
            if (tmpa == a && tmpb == b) {
                System.out.println(i);
                return;
            }
            
            if (tmpa > a && tmpb > b) {
                System.out.println(-1);
                return;
            }
        }
    }
}
/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