#include <bits/stdc++.h>
template <class T>
inline T lowbit(T x) {
  return x & (-x);
}
template <class T>
inline T sqr(T x) {
  return x * x;
}
template <class T>
inline bool scan(T &ret) {
  char c;
  int sgn;
  if (c = getchar(), c == EOF) return 0;
  while (c != '-' && (c < '0' || c > '9')) c = getchar();
  sgn = (c == '-') ? -1 : 1;
  ret = (c == '-') ? 0 : (c - '0');
  while (c = getchar(), c >= '0' && c <= '9') ret = ret * 10 + (c - '0');
  ret *= sgn;
  return 1;
}
const double pi = 3.14159265358979323846264338327950288L;
using namespace std;
int n;
int main() {
  cin >> n;
  cout << (n - 1) / 2 << endl;
  return 0;
}
