#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
using namespace std;
using ll = long long;
using pii = pair<int, int>;

int main() {
  int l, k;
  cin >> l >> k;
  k *= 2;
  if (l % k == 0) l -= k;
  else l -= l % k;
  int ans = l / 2;
  cout << ans << endl;
  return 0;
}