#include<iostream>
#include<vector>
#include<cstring>
#include<algorithm>
#include<cstdio>
#include<queue>
#include<string.h>
#include <string>
#include<math.h>
#include<cmath>
#include<set>
#include<unordered_map>
#include<unordered_set>
#include<map>
#include<queue>
#include<stack>
#include<functional>
#include<deque>
#define int long long
#define IOS ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define inf 1e18
using namespace std;
const int N = 2e5 + 7;
const int M = 1e6 + 7;
struct node
{
	int x;
	int y;
};
void solve()
{
	int n;
	cin >> n;
	vector<node>a(2*n + 1);
	bool fl = 0;
	for (int i = 1; i <= n; i++)
	{
		cin >> a[i+n].x >> a[i+n].y;
		a[i].x = a[i + n].x;
		a[i].y = a[i + n].y;
		if (a[i].y != a[i - 1 ? 1 : i - 1].y)fl = 1;
	}
	if (!fl)
	{
		cout << 0 << endl;
		return;
	}
	int cnt = 0;
	vector<bool>vi(n + 1,0);
	for (int i = n + 1; i <= 2 *n; i++)
	{
		if (vi[i %n])continue;
		int l, r;
		l = r = i;
		while (a[l].y == a[i].y)
		{
			l--;
		}
		while (a[r].y == a[i].y)
		{
			vi[r % n] = 1;
			r = (r) % (2 * n) + 1;
		}
		if (a[r].x < a[i].x && a[i].x < a[l].x);
		else if (a[l].y > a[i].y && a[r].y > a[i].y)
		{
			cnt++;
			vi[i % n] = 1;
		}

	}
	cout << cnt << endl;
}
signed main() {
	IOS;
	int t = 1; //cin >> t;
	while (t--) {
		solve();
	}
}