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

using namespace std;

int main() {
	int l, k;

	cin >> l >> k;
	int ans = l / (2 * k) * k;
	if (l % (2 * k) == 0) {
		ans -= k;
	}

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