#include <iostream>
#include <sstream>
#include <vector>
#include <algorithm>
#include <string>
#include <functional>
#include <numeric>
#include <ctype.h>
#include <cstdlib>
#include <string.h>
#include <cmath>
#include <cstdio>
#include <map>
#include <queue>
#include <utility>

using namespace std;

const int INF = 1000000007;
const double PI = acos(-1.0);


int main()
{
	int L, K;
	cin >> L >> K;
	int turn = (L - 1) / (2 * K);
	cout << turn * K << endl;
	
}