#include <bits/stdc++.h>
#define REP(i, a, n) for(ll i = ((ll) a); i < ((ll) n); i++)
using namespace std;
typedef long long ll;

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