#include <bits/stdc++.h>

using namespace std;

typedef pair<int,int> P;
typedef pair<int,pair<int,int>> PP;
typedef long long ll;

const double EPS = 1e-8;
const int INF = 1e9;
const int MOD = 1e9+7;

int dy[] = {0,1,0,-1};
int dx[] = {1,0,-1,0};

int main(void) {
	int L,K;
	cin >> L >> K;

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

	return 0;
}