#include<iostream>
#include<vector>
#include<cstring>
#include<algorithm>
#include<cstdio>
#include<queue>
#include<string.h>
#include <string>
#include<math.h>
#include<set>
#include<unordered_map>
#include<unordered_set>
#include<map>
#include<queue>
#include<stack>
#include<functional>
#include<deque>
using namespace std;

#define MAXN 301000;
#define ll long long
#define lll unsigned long long
#define PA pair<ll,ll>
#define INF (ll)0x3f3f3f3f*(ll)1000000
#define IOS ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);

const ll mod = 1e9 + 7;

struct node {
	int x, y;
};

void solve() {
	int n; cin >> n;
	vector<node>arr(2 * n);

	bool c = false;
	for (int i = 0; i < n; i++) {
		cin >> arr[i].x >> arr[i].y;
		arr[i + n] = arr[i];

		if (arr[i + n].y != arr[i - 1 + n].y)c = true;
	}

	if (!c) {
		cout << 0 << endl;
		return;
	}

	vector<bool>visit(n, false);
	int cnt = 0;
	for (int i = n; i < 2 * n; i++) {
		if (visit[i % n])continue;

		int l = i - 1, r = (i + 1) % (2 * n);
		while (arr[l].y == arr[i].y)l--;
		while (arr[r].y == arr[i].y)r = (r + 1) % (2 * n);

		int p = r - 1;
		if (p < 0)p = 2 * n - 1;
		while (p != l) {
			visit[p % n] = true;
			p--;
			if (p < 0)p = 2 * n - 1;
		}
		if (arr[i].y < arr[l].y && arr[i].y < arr[r].y)
		{
			if (arr[l].x <= arr[r].x)
			{

				double k1;
				double k2;
				if ((arr[l].x - arr[i].x) == 0)k1 = 1;
				else k1 = (double)(arr[l].y - arr[i].y) / (arr[l].x - arr[i].x);
				if ((arr[r].x - arr[i].x) == 0)k2 = 1;
				else k2 = (double)(arr[r].y - arr[i].y) / (arr[r].x - arr[i].x);

				if (arr[l].x <= arr[i].x && arr[i].x <= arr[r].x)cnt++;
				else if (k1 > k2)cnt++;
			}
			else
			{
				double k1;
				double k2;
				if ((arr[l].x - arr[i].x) == 0)k1 = 1;
				else k1 = (double)(arr[l].y - arr[i].y) / (arr[l].x - arr[i].x);
				if ((arr[r].x - arr[i].x) == 0)k2 = 1;
				else k2 = (double)(arr[r].y - arr[i].y) / (arr[r].x - arr[i].x);
				if (k1 > k2)
				{
					if (arr[r].x <= arr[i].x && arr[i].x <= arr[l].x);
					else cnt++;
				}
			}
		}
		

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