import java.util.*;

public class Main {
    public static void main(String[] args) {
        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);
        }
    }
}