#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <math.h>
int main(void)
{
    int num,fune,ans;
    scanf("%d %d",&num,&fune);
    if(fune>=num)
    {
        ans=1;
    }
    else if (num%2==0&&fune>=num/2)
    {
        ans=2;
    }
    else
    {
        ans=-1;
    }
    printf("%d",ans);
    
}