#include <iostream>
using namespace std;

int main()
{
	unsigned int L, K;
	unsigned int i = 0;

	cin >> L >> K;

	while (1)
	{
		if (L <= 2 * K)
		{
			break;
		}

		L -= 2 * K;
		i++;
	}
	cout << K*i;

}