#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
#include <iostream>
#include <map>
#include <list>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <string>
#include <vector>
#include <algorithm>
#include <utility>
#include <queue>
#include <iomanip>
#define INF 1050000000
#define SIZE 4
#define MOD 1000000007

using namespace std;

int main(void)
{
	int l, k;

	cin >> l >> k;

	cout << ((l - 1) / (2 * k)) * k << endl;

	return (0);
}