#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>

using namespace std;

int main() {
	int l, k;

	cin >> l >> k;

	int k2 = k * 2;
	int ans = 0;
	while ( l > k2) {
		l -= k2;
		ans += k;
	}

	cout << ans << endl;
	return 0;
}