#include<iostream>
#include<cstdio>
#include<vector>
using namespace std;

int main() {
	int l, k;
	cin >> l >> k;

	int ans = 0;
	for (int i = l; (i-2*k) > 0; i-=(2*k))
	{
		ans += k;
	}
	cout << ans << endl;

	return 0;
}