#include <bits/stdc++.h>
using namespace std;
#define REP(i,n) for (int i = 0; i < n; i++)
typedef long long ll;
// Welcome to my source code!

int main() {
    int l, k;
    cin >> l >> k;
    int cnt = 0, rem = l;
    REP(i, l/(k*2)) {
        if (rem - k*2 > 0) {
            rem -= k*2;
            cnt++;
        }
    }
    cout << cnt*k << endl;
}