#include<bits/stdc++.h>

#define endl '\n'
using ll = long long;
using ld = long double;
using namespace std;

bool turnleft(pair<ll, ll> p1, pair<ll, ll> p2, pair<ll, ll> p3){
    auto [x1, y1] = p1;
    auto [x2, y2] = p2;
    auto [x3, y3] = p3;
    ll xa = 0, ya = 0, xb = 0, yb = 0;
    xa = x2 - x1;
    ya = y2 - y1;
    xb = x3 - x2;
    yb = y3 - y2;
    return xa * yb - xb * ya >= 0;
}
//
//void solve(){
//    int n;
//    cin >> n;
//
//    vector<pair<int, int> > a(n + 1);
//    for (int i = 0; i < n; i++){
//        cin >> a[i].first >> a[i].second;
//    }
//    int st=0;
//    for(int i=0;i<n;i++){
//        if(a[i].second>a[st].second){
//            st=i;
//            //break;
//        }
//    }
//    vector<pair<int, int> > point;
//    point.push_back({a[st]});
//    point.push_back({a[(st+1)%n]});
//    for (int j = 2; j < n; j++){
//        int i=(j+st)%n;
//        if (a[i].second >= point.back().second && point[point.size() - 1].second >= point[point.size() - 2].second&&a[i].first >= point.back().first && point[point.size() - 1].first >= point[point.size() - 2].first){
//            point.back() = a[i];
//            continue;
//        }
//        else{
//            point.push_back({a[i]});
//        }
//    }
//
////    cout << "point:" << endl;
////    for (auto i: point){
////        cout << i.first << " " << i.second << endl;
////    }
//    int si = point.size();
//    int ans = 0;
//    for (int i = 0; i < si; i++){
//        int pr = (i - 1 + si) % si;
//        int ne = (i + 1) % si;
//
//        if (point[i].second <= point[pr].second && point[i].second < point[ne].second &&
//            turnleft(point[pr], point[i], point[ne])){
//            //cout << i << endl;
//            ans++;
//        }
//    }
//    cout << ans << endl;
//}

 void solve() {
     vector <pair <ll,ll>> point;
     ll n;cin>>n;
     vector <pair <ll,ll>> p(n+5);
     for(ll i=3;i<=n+2;i++) {
         cin>>p[i].first>>p[i].second;
     }
     p[1]=p[n+1];
     p[2]=p[n+2];
     p[n+3]=p[3];
     p[n+4]=p[4];
     for(ll i=1;i<=n+4;i++) {
         ll j=i+1;
         for(;j<=n+4;j++) {
             if(p[j].second!=p[i].second) break;
         }
         point.push_back(p[i]);
         if(j!=i+1) point.push_back(p[j-1]);
         i=j-1;
     }
     vector <pair <ll,ll>> check_point;
     check_point.push_back({0,0});
     for(auto t:point) check_point.push_back(t);
     auto check_min=[&](ll left,ll right)->bool {
         if(check_point[left].second == check_point[right].second) {
             return (check_point[left-1].second>check_point[left].second && check_point[right+1].second >check_point[right].second);
         }return false;
     };
     ll ans=0;
     for(ll i=3;i<=2+n;i++) {
         if(check_min(i,i+1) && turnleft(check_point[i-1],check_point[i],check_point[i+2])) {
             ans++;
             //cout<<"be_chosen_point:: "<<check_point[i].first<<' '<<check_point[i].second<<endl;
         }
     }
     for(ll i=3;i<=2+n;i++) {
         if(check_point[i-1].second>check_point[i].second && check_point[i+1].second > check_point[i].second
             &&turnleft(check_point[i-1],check_point[i],check_point[i+1])
             ) {
             //cout<<"i::"<<i<<endl;
             //cout<<"be_chosen_point:: "<<check_point[i].first<<' '<<check_point[i].second<<endl;
             ans++;
         }
     }
     //cout<<"Get"<<endl;
      for(auto t:check_point) {
          //cout<<t.first<<' '<<t.second<<endl;
      }
     cout<<ans<<endl;
 }

int main()
{
    ios::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
    int ttt = 1;
    //cin >> ttt;
    while (ttt--)
    {
        solve();
    }
    return 0;
}