#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int ans = 0;
  if (n % 2 == 1) {
    for (int i = 1; i <= n; i += 2) ans += i;
  } else {
    for (int i = 2; i <= n; i += 2) ans += i;
  }
  cout << ans << endl;
  return 0;
}
