#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
typedef long long ll;
const int INF = 1000000000; // 10^9
int main() {
    // ifstream in("data1.txt");
    // cin.rdbuf(in.rdbuf());
    int L, K;
    cin >> L >> K;
    int twobite = K * 2;
    int bited_cnt = L / twobite;
    if(L % twobite == 0) {
        cout << bited_cnt - 1 << endl;
        return 0;
    }
    int ans = bited_cnt * K;
    cout << ans << endl;
}