#include <iostream>

int main(int argc, char *argv[]) {
	int pocky_length = 0;
	int eat_length = 0;

	std::cin >> pocky_length;
	std::cin >> eat_length;

	int count = 0;
	while (1) {
		pocky_length -= (eat_length * 2);
		count++;
		if (pocky_length <= 0) {
			break;
		}
	}

	std::cout << (eat_length * (count - 1)) << std::endl;
	return 0;
}