#include <iostream>
#include <vector>
#include <set>
#include <cassert>
using namespace std;
using pii = pair<int, int>;

const int mn = 2e3 + 5;
pii poly[3*mn];

set<int> drain1, drain2;

pii sub(pii a, pii b) {
	return {a.first - b.first, a.second - b.second};
}

int cross(pii l, pii r) {
	return l.first * r.second - r.first * l.second;
}

int main() {
	int n; cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> poly[i].first >> poly[i].second;
		poly[i+n] = poly[i];
		poly[i+2*n] = poly[i];
	}
	
	int cnt = 0;
	int lo1 = 1e9, lo2 = 1e9;
	for (int i = n; i < 2*n; i++) {
		if (poly[i-1].second > poly[i].second && poly[i+1].second >= poly[i].second) {
			while (poly[i+1].second == poly[i].second) i++;
			int dir = cross(sub(poly[i-1], poly[i]), sub(poly[i+1], poly[i]));
			assert(dir != 0);
			if (poly[i+1].second > poly[i].second) {
				if (dir > 0) {
					drain1.insert(i%n);
					lo1 = min(lo1, poly[i].second);
				}
				else if (dir < 0) {
					drain2.insert(i%n);
					lo2 = min(lo2, poly[i].second);
				}
			}
		}
	}
	if (lo2 < lo1) swap(drain1, drain2);
	cout << drain1.size() << endl;
}