#include <iostream>

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)

using namespace std;

int main() {
    int L, K, yuu = 0;

    cin >> L >> K;

    while (true) {
        L -= 2 * K;
        if (L > 0) {
            yuu += K;
        } else {
            break;
        }
    }

    cout << yuu << endl;

    return 0;
}