import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Scanner;

public class Main {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final long n = sc.nextLong();
		final long m = sc.nextLong();
		
		if(n == m){
			System.out.println(1);
		}else if(n % 2 == 1){
			System.out.println(-1);
		}else if(n / 2 > m){
			System.out.println(-1);
		}else{
			System.out.println(2);
		}
	}
}