#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < (n); i++)
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const int INF = 1001001001;
const ll LINF = 1001002003004005006ll;
//const int mod = 1000000007;
//const int mod = 998244353;

int main()
{
  ll n;
  cin >> n;
  if (n <= 2) cout << 0 << endl;
  else if (n&1) cout << n/2 + 1 << endl;
  else cout << n/2 << endl;
  return 0;
}