#include <bits/stdc++.h>
const double eps = (1e-10);
using namespace std;
int dcmp(long double a, long double b) {
  return fabsl(a - b) <= eps ? 0 : (a > b) ? 1 : -1;
}
long long getBit(int num, int idx) { return ((num >> idx) & 1) == 1; }
long long setBit1(int num, int idx) { return num | (1 << idx); }
long long setBit0(long long num, int idx) { return num & ~(1ll << idx); }
long long flipBit(long long num, int idx) { return num ^ (1ll << idx); }
void M() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
int countNumBit1(int mask) {
  int ret = 0;
  while (mask) {
    mask &= (mask - 1);
    ++ret;
  }
  return ret;
}
vector<pair<pair<int, int>, pair<int, int> > > v;
vector<pair<int, pair<pair<int, int>, pair<int, int> > > > v1;
int getkey(int x, int y, int strx, int stry) {
  int key;
  if (strx == x) {
    if (stry > y)
      key = 2;
    else
      key = 1;
  } else if (stry == y) {
    if (strx > x)
      key = 3;
    else
      key = 4;
  }
  return key;
}
int main() {
  int n;
  cin >> n;
  int x, y;
  cin >> x >> y;
  int x1, y1, x2, y2, stop = -100000;
  for (int i = 1; i <= n / 2; i++) {
    cin >> x1 >> y1 >> x2 >> y2;
    v.push_back(make_pair(make_pair(x1, y1), make_pair(x2, y2)));
    stop = max(stop, max(x1, x2));
  }
  for (int i = 0; i < ((int)(v).size()); i++) {
    int strx = v[i].first.first;
    int stry = v[i].first.second;
    int enx = v[i].second.first;
    int eny = v[i].second.second;
    int nsx = x;
    int nex = strx;
    int nsy = y;
    int ney = stry;
    int key1 = getkey(x, y, strx, stry);
    int key2 = getkey(strx, stry, enx, eny);
    v1.push_back(
        make_pair(key1, make_pair(make_pair(nsx, nsy), make_pair(nex, ney))));
    v1.push_back(
        make_pair(key2, make_pair(make_pair(strx, stry), make_pair(enx, eny))));
    x = enx;
    y = eny;
  }
  int cnt = 0;
  for (int i = 0; i < ((int)(v1).size()) - 1; i++) {
    int cur = v1[i].first;
    int nxt = v1[i + 1].first;
    if ((cur == 3 && nxt == 2) || (cur == 4 && nxt == 1) ||
        (cur == 2 && nxt == 4) || (cur == 1 && nxt == 3))
      cnt++;
  }
  cout << cnt << endl;
}
