#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
using ll = long long;
using namespace std;
const int INF = 1e9;

int main() {
    int l, k;
    cin >> l >> k;

    cout << (l - 1) / (2 * k) * k << endl;

    return 0;
}