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();
    int ans = -1;
    if(n % 2 == 1) {
      if(m >= n) ans = 1;
    } else {
      if(m >= n) {
        ans = 1;
      } else {
        if(m >= (n / 2)) ans = 2;
      }
    }
    System.out.println(ans);
  }
}