#include <bits/stdc++.h>
using namespace std;

using ll = long long;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define rep2(i, l, r) for (ll i = l; i < r; i++)

using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;

#define all(A) A.begin(), A.end()
#define elif else if
using pii = pair<ll, ll>;

bool chmin(auto &a, const auto &b) { return a > b ? a = b, 1 : 0; }
bool chmax(auto &a, const auto &b) { return a < b ? a = b, 1 : 0; }

struct IOSetup {
  IOSetup() {
    cin.tie(0);
    ios::sync_with_stdio(0);
  }
} iosetup;

template <class T>
void print(vector<T> a) {
  for (auto x : a) cerr << x << ' ';
  cout << endl;
}

void print(auto x) { cout << x << endl; }

template <class Head, class... Tail>
void print(Head &&head, Tail &&...tail) {
  cout << head << ' ';
  print(forward<Tail>(tail)...);
}

using Real = double;
using Point = complex<Real>;
using Poly = vector<Point>;
Real eps = 1e-9;

int Round(Real x) { return static_cast<int>(round(x)); }

// 同じか判定
bool equal(Point a, Point b) { return abs(a - b) < eps; }

// 内積(dot product): a・b=|a||b|cosΘ
Real dot(Point a, Point b) { return a.real() * b.real() + a.imag() * b.imag(); }
// 外積(cross product): a×b=|a||b|sinΘ
Real cross(Point a, Point b) { return a.real() * b.imag() - a.imag() * b.real(); }
Point unitVector(Point p) { return p / abs(p); }

// 多角形 g に点 p が含まれるか? 含む:2, 辺上:1, 含まない:0
int isContain(Poly g, Point p) {
  int IN = 0;
  int n = g.size();
  rep(i, n) {
    Point a = g[i] - p;
    Point b = g[(i + 1) % n] - p;
    if (a.imag() > b.imag()) swap(a, b);
    if (imag(a) < eps && eps < imag(b) && cross(a, b) < -eps) IN ^= 1;
    if (equal(cross(a, b), 0) && dot(a, b) < eps) return 1;
  }
  return 2 * IN;
}


int main(){
  int n;
  cin>>n;
  vi x(n),y(n);
  Poly g;
  rep(i,n){
    cin>>x[i]>>y[i];
    y[i]*=100;
    g.push_back(Point((Real)x[i],(Real)y[i]));
  }
  int ans=0;
  rep(i,n){
    if(y[(i-1+n)%n]<=y[i])continue;
    int now=(i+1)%n;
    while(y[i]==y[now]){
      now++;
      now%=n;
    }
    if(y[now]>y[i]){
      Point p1=(g[(i-1+n)%n]-g[i]);
      Point p2=(g[(i+1)%n]-g[i]);
      Point q=g[i]+(p1+p2)*1e-4;
      if(isContain(g,q)>=1)ans++;
    }
  }
  cout<<ans<<endl;
}