#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <numeric>
#include <functional>
#include <set>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cctype>
#include <climits>

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define EXIST(v, n) (std::find(v.begin(), v.end(), 1) != v.end())

using namespace std;

int main(int argc, char *argv[]) {
	long long L, K, ans = 0;
	cin >> L >> K;
	while (L) {
		if (L - 2 * K <= 0) {
			break;
		}
		ans += K;
		L -= 2 * K;
	}
	cout << ans << endl;
}