#include"bits/stdc++.h"
using namespace std;
using ll = long long;
using ld = long double;
#define rep(i,m,n) for(ll i=(ll)m;i<(ll)n;i++)
#define Endl endl
#define pr(i,j) make_pair(i,j) 
const ll mod = 998244353;
const ll inf = 5e18;
const ld pi = 3.14159265358979;
int main() {
	ll l, k;
	cin >> l >> k;
	ll s = l % (2 * k);
	if (s == 0)s += 2 * k;
	ll ans = (l - s) / 2;
	cout << ans << endl;
}