#include<stdio.h>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <string>

using namespace std;

int main() {

	int L, K;

	cin >> L >> K;

	int res = L, cnt = 0;;
	while (1) {
		if (res - (2 * K) <= 0) break;
		res -= 2 * K;
		cnt++;
	}
	
	cout << K*cnt << endl;

	return 0;
}