#pragma GCC optimize(3, "Ofast", "inline")
#include <iostream>
#include <bits/stdc++.h>
#define IOS ios::sync_with_stdio(false), cin.tie(0), cout.tie(0)
#define INF 0x3f3f3f3f
#define L_INF 0x7f3f3f3f3f3f3f3f
#define db cout << "debug\n";

using namespace std;
const int Mod = 998244353;
using ll = long long;
int cross(pair<int, int> s1, pair<int, int> s2)
{
    return s1.first * s2.first + s2.second * s2.second;
}
void solve()
{
    int n;
    cin >> n;
    vector<pair<int, int>> p(n + 3);
    for (int i = 1; i <= n; i++)
    {
        int x, y;
        cin >> x >> y;
        p[i] = {x, y};
    }
    p[n + 1] = p[1];
    p[n + 2] = p[2];
    vector<int> vis(n + 3, 0);
    vector<pair<int, int>> kt(n + 3);
    for (int i = 1; i <= n + 1; i++)
    {
        kt[i] = {p[i + 1].first - p[i].first, p[i + 1].second - p[i].second};
    }
    // {-y,x}
    int ans = 0;

    for (int i = 2; i <= n + 1; i++)
    {
        if (kt[i - 1].second <= 0 && kt[i].second > 0) // 向下  (x,y)
        {
            pair<int, int> tmp = {-kt[i - 1].second, kt[i - 1].first};
            if (cross(tmp, kt[i]) > 0)
                ans++;
        }
    }
    cout << ans << '\n';
}
int main()
{
    IOS;
// freopen("in.txt", "r", stdin);
// freopen("out.txt", "w", stdout);
#ifndef ONLINE_JUDGE
    clock_t start_time = clock();
#endif
    int t = 1;
    // cin >> t;
    while (t--)
    {
        solve();
    }
#ifndef ONLINE_JUDGE
    cout << "Used " << (double)(clock() - start_time) << " ms" << endl;
#endif
    return 0;
}