10
-2 1
2 -2
-1 -2
0 0
2 -1
0 -2
2 2
0 2
-1 -1
1 -2
 40
-8 24
2 -1
1 -18
72 -70
5 -4
-308 436
-19 40
36 -35
-178 265
-1 2
-7 30
-1 0
3 -2
200 -285
17 -16
-35 74
0 -4
-86 106
-1 4
-7 6
0 1
-5 4
-2 3
6 -5
-4 5
181 -262
76 -118
0 0
-7 18
-58 104
-5 6
-6 12
-3 4
1 0
11 -10
-86 130
-3 6
153 -236
-183 270
-33 64
 5
0 -1
0 -2
0 -3
0 -4
0 -5
 10
0 1
-1 -1
1 1
-1 0
1 -1
-2 -1
-2 2
-2 0
0 -2
0 -1
 20
-1 18
-2 5
-5 4
2 -33
9 -18
0 0
11 -22
2 0
-1 2
-4 41
1 6
1 -2
6 -12
0 1
-3 6
3 -6
3 -8
-1 4
2 -5
1 0
 10
2 1
-1 0
-2 -1
-1 1
0 2
2 -2
0 0
-2 -2
0 -2
-2 1
 20
-3 -3
0 4
-3 1
1 1
-1 2
-4 4
3 -1
-3 0
0 2
4 0
2 3
2 4
4 -3
-4 3
-1 1
1 3
-2 4
1 -2
1 -1
3 0
 3
0 0
0 -1
0 1
 10
2 1
-1 1
0 0
-3 1
-2 -3
-1 -2
-1 -1
1 2
3 -2
0 -2
 3
3 3
1 2
1 1
 4
0 0
1 0
2 0
3 0
 9
1 1
1 0
0 1
0 0
-1 0
-1 1
-1 -1
1 -1
0 -1
 7
1 1
-1 -1
1 0
0 1
-1 0
0 -1
0 0

4
 57
 4
 5
 21
 4
 10
 1
 1
 0
 2
 8
 3

#include <bits/stdc++.h>
const int N = 1e6 + 5;
using namespace std;
long long absolute(long long x) {
  if (x < 0)
    return -x;
  else
    return x;
}
bool comp(int a, int b) { return (a < b); }
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long a, long long b) { return (a * b) / (gcd(a, b)); }
void IOS() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
}
void seive() {
  long long IM;
  long long prime[IM + 1];
  long long i;
  for (i = 2; i * i <= IM; i++) {
    if (!prime[i]) {
      for (long long j = i * i; j <= N; j += i) {
        prime[j] = 1;
      }
    }
  }
}
void solve() {
  long long n, count = 0;
  cin >> n;
  vector<pair<int, int>> v;
  for (int i = 0; i < n; i++) {
    long long x, y;
    cin >> x >> y;
    v.push_back(make_pair(x, y));
  }
  for (int i = 0; i < v.size() - 2; i++) {
    if ((v[i + 1].first == (v[i].first + v[i + 2].first) / 2) &&
        (v[i + 1].second == (v[i].second + v[i + 2].second) / 2))
      count++;
    long long x = v.size() - 1;
    if ((v[i].first == (v[0].first + v[x].first) / 2) &&
        (v[i].second == (v[0].second + v[x].second) / 2))
      count += 1;
  }
  cout << count << '\n';
}
int32_t main() {
  IOS();
  solve();
  return 0;
}
