import java.math.BigInteger;
import java.util.Arrays;
import java.util.Scanner;

class Main {
	public static void main(String[] args) {
		long t = System.currentTimeMillis();
		new Main().run();
		// new Thread(null, new Main(), "",
		// Runtime.getRuntime().maxMemory()).start();
		System.err.println(System.currentTimeMillis() - t);
	}

	@SuppressWarnings("unchecked")
	public void run() {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		if (n <= m) {
			System.out.println(1);
		} else if (n % 2 == 0 && n / 2 <= m) {
			System.out.println(2);
		} else {
			System.out.println(-1);
		}
	}

	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
}