#include <iostream>

using  namespace std;

int main(int argc, char const* argv[]) {
    int l, k;
    cin >> l >> k;
    int ans;
    if (l % (2 * k) == 0) {
        ans = (l / (2 * k) - 1) * k;
    } else {
        ans = l / (2 * k) * k;
    }
    cout << ans << endl;
    return 0;
}