#include<iostream>
#include<vector>
#include<algorithm>
#include<set>
#include<iomanip>
#include<queue>
#include<math.h>
#include<map>
#include<bitset>
#include<numeric>
#include <cstring>
#include<array>
#include <string>  
#include<unordered_set>
using namespace std;
#define int long long
#define endl "\n"
#define mod 1000000007ll
#define IOS  ios::sync_with_stdio(false),cin.tie(nullptr)
#define syy return
const double pi = acos(-1.0);
using ll = long long;
using pii = pair<int, int>;
using vint = vector<int>;
using vll = vector<ll>;
using vvint = vector<vint>;
using vpo = vector<pii>;
using tup = tuple<int, int, int>;
using vbl = vector<bool>;
constexpr int N = 500005, M = 2000006;//开足够的空间 注意无向图的双向边边数*2
constexpr ll inf = 3e18 + 5000;
long long cross(long long x1, long long y1, long long x2, long long y2) {
	return x1 * y2 - x2 * y1;
}
void slove() {
	int n; cin >> n;
	vpo a(n);
	vpo b;
	for (auto& [x, y] : a)cin >> x >> y;
	for (int i = 0; i < n; i++) {
		int j = (i - 1 + n) % n;
		int k = (i + 1) % n;
		auto [x1, y1] = a[i];
		auto [x0, y0] = a[j];
		auto [x2, y2] = a[k];

		if ((x0 == x1 and x1 == x2) or (y1 == y0 and y1 == y2))
		{
			i++;
			continue;
		}
		b.push_back(a[i]);
	}
	n = b.size();
	//cout << n << endl;
	int ans = 0;
	for (int i = 0; i < n; i++) {
		int j = (i - 1 + n) % n;
		int k = (i + 1) % n;
		int z = (i + 2) % n;
		auto [x0, y0] = b[j];
		auto [x1, y1] = b[i];
		auto [x2, y2] = b[k];
		auto [x3, y3] = b[z];
		if (y1 == y2 and y1 < y0 and y1 < y3)++ans;
		else if (y0>y1 and y1< y2 and cross(x1 - x0, y1 - y0, x2 - x1, y2-y1) > 0) ans++;

	}
	cout << ans << endl;
}


signed main()
{
	IOS;
	int T = 1;
	//cin >> T;
	while (T--) {
		slove();
	}
	return 0;
}