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

int main() {
    int L, K;
    cin >> L >> K;

    cout << (L-1)/(2*K)*K << endl;
}