#include <iostream>
#include <sstream>
#include <string>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <vector>
#include <array>
#include <map>
#include <memory>
#include <algorithm>
#include <utility>

namespace Util
{

std::vector<std::string> Split(const std::string& str, const char delim);
std::string Itos(const int value);
std::string Ftos(const double value);
int Stoi(const std::string& buf);
double Stof(const std::string& buf);
std::vector<int> ConvertInt(const std::vector<std::string>& vecBuf);
std::vector<double> ConvertDouble(const std::vector<std::string>& vecBuf);

}// namespace Util


int main()
{
	// 入出力の速度向上
	std::cin.tie(0);
	std::ios::sync_with_stdio(false);

	std::string buf;
	std::vector<std::string> vecBuf;

	int length, eat;
	int chanYu;

	std::getline(std::cin, buf);
	vecBuf = Util::Split(buf, ' ');
	length = Util::Stoi(vecBuf[0]);
	eat = Util::Stoi(vecBuf[1]);

	chanYu = (length / eat) <= 2 ? 0 : (length / eat) / 2;
	printf("%d\n", chanYu * eat);


	return 0;
}


namespace Util
{

std::vector<std::string> Split(const std::string& str, const char delim)
{
	std::istringstream iss(str);
	std::vector<std::string> ret;
	std::string tmp;

	while(std::getline(iss, tmp, delim))
	{
		ret.push_back(tmp);
	}

	return ret;
}

std::string Itos(const int value)
{
	std::ostringstream ostream;
	ostream << value;
	return ostream.str();
}

std::string Itof(const double value)
{
	std::ostringstream ostream;
	ostream << value;
	return ostream.str();
}

int Stoi(const std::string& buf)
{
	return std::atoi(buf.c_str());
}

double Stof(const std::string& buf)
{
	return std::atof(buf.c_str());
}

std::vector<int> ConvertInt(const std::vector<std::string>& vecBuf)
{
	std::vector<int> ret(vecBuf.size());

	for(int cur = 0; cur < static_cast<int>(ret.size()); ++cur)
	{
		ret[cur] = Stoi(vecBuf[cur]);
	}

	return ret;
}

std::vector<double> ConvertDouble(const std::vector<std::string>& vecBuf)
{
	std::vector<double> ret(vecBuf.size());

	for(int cur = 0; cur < static_cast<int>(ret.size()); ++cur)
	{
		ret[cur] = Stof(vecBuf[cur]);
	}

	return ret;
}

}// namespace Util