#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <iomanip>
#include <cmath>
using namespace std;
#define diff(x,y) ((x > y) ? (x - y) : (y - x))
#define us(x) (x > 0) ? x : x * -1

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