#include <iostream>
#include <stdio.h>
#include <cmath>
#include <algorithm>
#include <vector>

#define FOR(i, a, b) for (int i = a; i < b; i++)
#define FORN(i, a, b) for (int i = a; i <= b; i++)
#define P(s) cout << s << endl
#define PC(condition, _true, _false) if (condition) _true; else _false;
#define toInt(_char) _char - '0'

using namespace std;
//
// Created by karayuu on 2018-12-18.
//

int main() {
    int l, k;
    cin >> l >> k;

    int shrinkage = k * 2;
    int count;
    if (l % shrinkage == 0) {
        count = l / shrinkage - 1;
    } else {
        count = l / shrinkage;
    }

    P(count * k);
}