#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) {
    cout << 0 << endl;
    return 0;
  }
  int ans = (l - l % k) / 2;
  cout << ans << endl;
  return 0;
}