#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(i64 (i) = (s);(i) < (e);(i)++)
#define all(x) x.begin(),x.end()


#include <type_traits>
#include <vector>
using i64 = long long;

template<class Iterator> class jter;
template<class Jter> class rev_jter;
template<class Jter, class F> class map_jter;

template<class Iterator>
class jter {

  Iterator b;
  Iterator e;

  public:

  using iterator = jter<Iterator>;
  using value_type = typename Iterator::value_type;
  using difference_type = std::ptrdiff_t;
  using pointer = value_type*;
  using reference = value_type&;
  using iterator_category = std::input_iterator_tag;

  jter() {}
  jter(Iterator b, Iterator e) : b(b), e(e) {}

  void next() { b++; }
  void prev() { --e; }
  value_type& front() { return *b; }
  value_type& back() { auto c = e; return *(--c); }
  bool isnone() const { return b == e; }

  jter begin() const { return *this; }
  jter end() const { return jter(); }
  jter& operator++() { next(); return *this; }
  value_type& operator*() { return front(); }
  bool operator!=(const jter& other) const { return !isnone(); }

  rev_jter<jter<Iterator>> rev() const { return rev_jter<jter<Iterator>>(*this); }
  template<class F>
    map_jter<jter<Iterator>, F> map(F f) const { return map_jter<jter<Iterator>, F>(*this, f); }
  std::vector<value_type> tovec() const { return std::vector<value_type>(this->begin(), this->end()); }

  template<class R, class F>
    R sum(R init, F f) const {
      for(const auto& x: *this) init = f(init, x);
      return init;
    }
};

template<class Jter>
class rev_jter {

  Jter j;

  public:

  using iterator = rev_jter<Jter>;
  using value_type = typename Jter::value_type;
  using difference_type = std::ptrdiff_t;
  using pointer = value_type*;
  using reference = value_type&;
  using iterator_category = std::input_iterator_tag;

  rev_jter() {}
  rev_jter(Jter j): j(j) {}

  void next() { j.prev(); }
  void prev() { j.next(); }
  value_type& front() { return j.back(); }
  value_type& back() { return j.front(); }
  bool isnone() const { return j.isnone(); }

  rev_jter begin() const { return *this; }
  rev_jter end() const { return rev_jter(); }
  rev_jter& operator++() { next(); return *this; }
  value_type& operator*() { return front(); }
  bool operator!=(const rev_jter& other) const { return !isnone(); }

  Jter rev() const { return j; }
  template<class F>
    map_jter<rev_jter, F> map(F f) const { return map_jter<rev_jter, F>(*this, f); }
  template<class R, class F>
    R sum(R init, F f) const {
      for(const auto& x: *this) init = f(init, x);
      return init;
    }
};

template<class Jter, class F>
class map_jter {

  Jter j;
  union _u {
    F f;
    bool isend;
    _u(): isend(true) {} 
    _u(F f): f(f) {}
  } u;

  public:

  using iterator = rev_jter<Jter>;
  using value_type = decltype(std::declval<F>()(std::declval<typename Jter::value_type>()));
  using difference_type = std::ptrdiff_t;
  using pointer = value_type*;
  using reference = value_type&;
  using iterator_category = std::input_iterator_tag;

  map_jter() {}
  map_jter(Jter j, F f): j(j), u(f) {}

  void next() { j.next(); }
  void prev() { j.prev(); }
  value_type front() { return u.f(j.front()); }
  value_type back() { return u.f(j.back()); }
  bool isnone() const { return j.isnone(); }

  map_jter begin() const { return *this; }
  map_jter end() const { return map_jter(); }
  map_jter& operator++() { next(); return *this; }
  value_type operator*() { return front(); }
  bool operator!=(const map_jter& other) const { return !isnone(); }

  Jter rev() const { return j; }
  template<class G>
    map_jter<map_jter, G> map(F f) const { return map_jter<map_jter, G>(*this, f); }
  std::vector<value_type> tovec() const { return std::vector<value_type>(this->begin(), this->end()); }
  template<class R, class G>
    R sum(R init, G g) const {
      for(const auto& x: *this) init = g(init, x);
      return init;
    }
};

class range {

  i64 b;
  i64 e;

  public:

  using iterator = range;
  using value_type = i64;
  using difference_type = std::ptrdiff_t;
  using pointer = value_type*;
  using reference = value_type&;
  using iterator_category = std::input_iterator_tag;

  range() {}
  range(i64 n) : b(0), e(n) {}
  range(i64 b, i64 e) : b(b), e(e) {}

  value_type next() { return b++; }
  value_type prev() { return --e; }
  value_type front() { return b; }
  value_type back() { return e - 1; }
  bool isnone() const { return b == e; }

  range begin() const { return *this; }
  range end() const { return range(); }
  range& operator++() { next(); return *this; }
  value_type operator*() { return front(); }
  bool operator!=(const range& other) const { return !isnone(); }

  rev_jter<range> rev() const { return rev_jter<range>(*this); }

  template<class F>
    map_jter<range, F> map(F f) const { return map_jter<range, F>(*this, f); }

  std::vector<value_type> tovec() const { return std::vector<value_type>(this->begin(), this->end()); }
  template<class R, class F>
    R sum(R init, F f) const {
      for(const auto& x: *this) init = f(init, x);
      return init; }
};

template<class Iterator>
jter<Iterator> jtr(Iterator b, Iterator e) { return jter<Iterator>(b, e); }

template<class V>
jter<typename V::iterator> jtr(V& v) { return jter<typename V::iterator>(begin(v), end(v)); }

template<class V>
jter<typename V::const_iterator> cjtr(const V& v) { return jter<typename V::const_iterator>(cbegin(v), cend(v)); }

template<class T, class Arg = i64>
T input(Arg dummy = Arg()) { T t; std::cin >> t; return t; }

int main() {
  i64 N;
  cin >> N;
  i64 sum = N / 2;
  if(N % 2 == 0) sum--;
  cout << sum << endl;
}


Measured memory_bytes: 256