#include <iostream>
#include <vector>
#include <algorithm>
#include <math.h>
#include <map>

#define ALL(x) (x).begin(),(x).end()
#define RALL(x) (x).rbegin(),(x).rend()
#define REP(i,a,n) for(int i = (a);i < (n);i++)
#define PI 3.14159265359
#define MOD 1000000007

using namespace std;

int main(){
    int n,m,ans = -1;
    cin >> n >> m;
    if(n <= m)ans = 1;
    else if(n%2 == 0 && n/2 <= m)ans = 2;
    cout << ans << endl;
    return 0;
}