#include <bits/stdc++.h>
#define test(x) cerr<<#x<<'='<<x<<endl
#define endl '\n'
#define int long long
#define PUT putchar('\n')
using namespace std;
const int MAXN = 1e6 + 100;
int n, m;
string s;
int a[MAXN], tot;

struct pt {
	int x, y;
	friend pt operator-(const pt &p1, const pt &p2) {
		return {p1.x - p2.x, p1.y - p2.y};
	}
	friend pt operator+(const pt &p1, const pt &p2) {
		return {p1.x + p2.x, p1.y + p2.y};
	}
	friend int operator^(const pt &p1, const pt &p2) {
		return p1.x * p2.y - p1.y * p2.x;
	}
	friend int operator*(const pt &p1, const pt &p2) {
		return p1.x * p2.x + p1.y * p2.y;
	}
	friend bool operator!=(const pt &p1, const pt &p2) {
		return (p1.x != p2.x) || (p1.y != p2.y);
	}
} p[MAXN], q[MAXN];

void cul() {
	cin >> n;
	tot = 0;
	for (int i = 1; i <= n; ++i)
		cin >> p[i].x >> p[i].y;
//	for (int i = 1; i <= n; ++i)
//		if(p[i+1].y != p[i].y)
	p[++n] = p[1], p[++n] = p[2];
	int ans = 0, f = 0;
	pt id = {0, 0};
	for (int i = 1; i <= n; ++i) {
		pt u = p[i + 1] - p[i];
		if (u.y < 0)
			id = u;
		else if (u.y == 0)
			f = 1;
		else if (u.y > 0) {
			if ((id ^ u) > 0 || f == 1) {
				++ans;
//				test(i);
			}
			id = {0, 0};
			f = 0;
		}
	}
	cout << ans << endl;
}

signed main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	int tt = 1;
//	cin >> tt;
	while (tt--)
		cul();
}