#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; }