#include <iostream>

using namespace std;

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