#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <numeric>

using namespace std;
typedef long long int lint;

int main()
{
	int L, K;
	cin >> L >> K;

	if (L % (2 * K) == 0)
		cout << (L / (2 * K) - 1) * K << endl;
	else
		cout << (L / (2 * K)) * K << endl;
	return 0;
}