#include <bits/stdc++.h>
using namespace std;
int main() {
    int n, m;
    cin >> n >> m;
    int k = min(m, n / 2);
    if (k == 0) {
        cout << -1 << endl;
        return 0;
    }
    cout << (n + k - 1) / k << endl;
    return 0;
}