import java.util.*;
public class Test10 {
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    String[] line;
    line=sc.nextLine().split(" ");
    int n=Integer.parseInt(line[0]),m=Integer.parseInt(line[1]);
    int d=-1;
    if(n<=m) d=1;
    else if(n%2==0&&n<=m*2) d=2;
    System.out.println(d);
    sc.close();
  }
}