#include <bits/stdc++.h>
using namespace std;

#define rep(i,a,b) for(int i=a;i < b;i++)
#define all(x) begin(x),end(x)
#define sz(x) (int)(x.size())
typedef long long int ll;
typedef pair<ll,ll> pii;
typedef vector<int> vi;
#define fi first
#define se second
ll cross(pii a,pii b){
	return a.fi * b.se - a.se*b.fi;
}
int main()
{
	int n;
	cin>>n;
	vector<pii>a(n);
	int ans = 0;
	for(int i=0;i<n;i++)cin>>a[i].fi>>a[i].se;
	for(int i=0;i<n;i++){
		pii b = a[(i+n-1)%n];
		pii c = a[(i+1)%n];
		if(b.se > a[i].se && c.se >= a[i].se){ //if valley you must taleke
			//cout<<i<<'\n';
			if(cross({c.fi - a[i].fi,c.se - a[i].se},{b.fi - a[i].fi,b.se - a[i].se}) >= 0)ans++;
		}
		/*
		else if(b.se > a[i].se && c.se == a[i].se){
			if(cross({c.fi - a[i].fi,c.se - a[i].se},{b.fi - a[i].fi,b.se - a[i].se}) >= 0){
				int j = (i+1)%n;
				while(a[j].se == a[i].se){
					j++;
					j%n;
				}
				if(a[j].se > a[i].se)ans++;
			}
		}*/
	}
	cout<<ans<<'\n';
}