#include<bits/stdc++.h>
using namespace std;
typedef double ld;
typedef long long ll;
struct p2{
	ll x,y;
	friend p2 operator+(p2 a,p2 b){return {a.x+b.x,a.y+b.y};}
	friend p2 operator-(p2 a,p2 b){return {a.x-b.x,a.y-b.y};}
	friend p2 operator*(ll a,p2 b){return {a*b.x,a*b.y};}
	friend ll operator*(p2 a,p2 b){return a.x*b.x+a.y*b.y;}
	friend ll operator^(p2 a,p2 b){return a.x*b.y-a.y*b.x;}
	void rd(){cin>>x>>y;}
	void db(){cout<<x<<' '<<y<<'\n';}
	
};

void solve(){
	int n;cin>>n;
	vector<p2> v(n);
	for(int i=0;i<n;i++) v[i].rd();
	int i=0,res=0;
	while(v[(i-1+n)%n].y==v[i%n].y) i++; 
	for(int j=0;j<n;j++,i++){
		int y1,y2,y3;
		p2 a=v[i%n]-v[(i-1+n)%n];
		y1=v[(i-1+n)%n].y;
		y2=v[i%n].y;
		while(v[i%n].y==v[(i+1)%n].y) i++,j++;
		p2 b=v[(i+1)%n]-v[i%n];
		y3=v[(i+1)%n].y;
		
		if((a^b)>0 && y1>y2 && y3>y2){
			cerr<<i%n<<'\n';
			res++;
		}else if(a.x==0 && b.x==0 && a.y<0 && b.y>0){
			cerr<<i%n<<'\n';
			res++;
		}
	}
	cout<<res<<'\n';
}
int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	solve();
	
}