#include<iostream>

int main() {
	int length, e;
	std::cin >> length >> e;

	int count = 0;
	while (length != 0) {
		if (length <= e * 2) {
			length = 0;
		}else{
			length -= e * 2;
			count++;
		}
	}

	std::cout << count * e << std::endl;
	return 0;
}