#include <iostream>
//#include <vector>
//#include <algorithm>
//#include <string>
//#include <iomanip>

using namespace std;

int main()
{
	long long 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;
}