#include<cstdio>
#include <iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> Pr;

ll L, K;

int main()
{
	cin >> L >> K;
	ll ans = 0;
	while (L > K) {
		L -= K * 2;
		if(L > 0)
		ans += K;
	}
	cout << ans << endl;
	return	0;
}