#include <bits/stdc++.h>
using namespace std;
long long power(long long a, long long b) {
  long long res = 1ll;
  while (b > 0) {
    if (b % 2 != 0) res = (res * a) % 1000000007;
    a = (a * a) % 1000000007;
    b /= 2;
  }
  return res % 1000000007;
}
long long ncr(long long n, long long k) {
  if (k > n - k) k = n - k;
  long long pro = 1;
  for (long long i = 0; i < k; i++) {
    pro = (pro * (n - i)) % 1000000007;
    pro /= (i + 1);
  }
  return (pro) % 1000000007;
}
void swap(char *a, char *b) {
  char temp = *a;
  *a = *b;
  *b = temp;
}
void solve() {
  long long n;
  cin >> n;
  long long ans = ceil(n / 2.0);
  cout << ans - 1;
  return;
}
int main(void) {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long t;
  t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
