import java.util.*;
import java.lang.*;
 
public class Main {
    public static void main(String[] args) {

        Scanner sc  = new Scanner(System.in);

        int n= Integer.parseInt(sc.next());
        int m = Integer.parseInt(sc.next());
        if((m+n)%2==0)System.out.print(m/n);
        else System.out.print("-1");
        
        System.out.println();
    }

}