#include <algorithm>
#include <iostream>
#include <iomanip>
#include <cstring>
#include <string>
#include <vector>
#include <queue>
#include <cmath>
#include <stack>
#include <set>
#include <map>
typedef long long ll;
typedef unsigned int uint;
using namespace std;


int main() {
	int l, k;
	int eat = 0;
	cin >> l >> k;

	while (l - 2 * k > 0) {
		l -= 2 * k;
		eat += k;
	}

	cout << eat << endl;
}