import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		String[] str = s.nextLine().split(" ");
		int n = Integer.parseInt(str[0]);
		int m =Integer.parseInt(str[1]);
		if(m>=n){
			System.out.println(1);
		}else if(n/2<=m && n%2 == 0){
				System.out.println(2);
			}else {
			System.out.println(-1);
		}

	}

}