#include <iostream>

using namespace std;

int main() {
	int len, x;
	cin >> len >> x;
	if (len % 2 == 0 && (len / 2) % x == 0) {
		cout << ((len / 2) / x - 1)*x << endl;
	}
	else{
		cout << ((len / 2) / x) * x << endl;
	}

	return 0;
}