#include<iostream>
using namespace std;
int main() {
	int a, b;
	cin >> a >> b;
	if (a % (2 * b) != 0) {
		cout << (a - (a % (2 * b))) / 2 << endl;
	}
	else {
		cout << (a / (2 * b) - 1)*b << endl;
	}
}