import java.util.*;
import static java.lang.System.*;

public class Main {
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		int n = sc.nextInt();
		int m = sc.nextInt();
		if (n <= m) {out.println(1);}
		else if (m == 2) {
			if (n == 2) {
				out.println(1);
			}
			else if (n == 4) {
				out.println(2);
			}
			else {
				out.println(-1);
			}
		}
		else {
			if (n%2==1) {out.println(-1);}
			else if (n%2==0) {
				if (n/2 > m) {out.println(-1);}
				else {out.println(2);}
			}
		}
	}
}