#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <stdio.h>
#include <cstring>
#include <queue>
#include <stack>
#include <functional>
#include <math.h>
#include <iomanip>
using namespace std;
#define Would
#define you

int main() {
	long long a, b;
	cin >> a >> b;
	cout << (a - 1) / (2 * b)*b << endl;
}