import java.io.*;
import java.util.*;


public class Main_yukicoder531 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		int n = sc.nextInt();
		int m = sc.nextInt();

		if (n % 2 == 0) {
			if (m < n / 2) {
				pr.println(-1);
			} else if (m >= n) {
				pr.println(1);
			} else {
				pr.println(2);
			}
		} else {
			if (m >= n) {
				pr.println(1);
			} else {
				pr.println(-1);
			}
		}
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(System.in);
		pr = new Printer(System.out);

		solve();

		pr.close();
		sc.close();
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}