#include <bits/stdc++.h>

[[nodiscard]] static inline constexpr uint_fast32_t solve(const uint_fast32_t L, const uint_fast32_t K) noexcept
{
	return K * ((L - 1) / (K * 2));
}

int main()
{
	std::cin.tie(nullptr);
	std::ios::sync_with_stdio(false);

	uint_fast32_t L, K;
	std::cin >> L >> K;

	std::cout << solve(L, K) << '\n';
	return 0;
}