#include <iostream>
#include <vector>

using namespace std;

pair<int,int> a[1000001];
vector<pair<int,int>> b;

signed main()
{
	ios::sync_with_stdio(false),cin.tie(0),cout.tie(0);
	
	int n;
	
	cin>>n;
	for(int i=0;i<n;i++){
		cin>>a[i].first>>a[i].second;
	}
	
	for(int i=0;i<n;i++){
		int dx1=a[i].first - a[(i+n-1)%n].first;
		int dy1=a[i].second - a[(i+n-1)%n].second;
		int dx2=a[(i+1)%n].first - a[i].first;
		int dy2=a[(i+1)%n].second - a[i].second;
		
		b.push_back(a[i]);
		
		if(dx1*dy2 == dx2*dy1)
		{
			b.pop_back();
		}
	}
	
	int ans=0;
	n=(int)b.size();
	for(int i=0;i<n;i++)
	{
		if(b[i].second < b[(i+1)%n].second && b[i].second < b[(i-1+n)%n].second)
		{
			int dx1=b[i].first - b[(i+n-1)%n].first;
			int dy1=b[i].second - b[(i+n-1)%n].second;
			int dx2=b[(i+1)%n].first - b[i].first;
			int dy2=b[(i+1)%n].second - b[i].second;
			
			if(dx1==0)
			{
				if(dx2*dy2>0) ans++;
			}
			else if(dx2==0)
			{
				if(dx1*dy1<0) ans++;
			}
			else
			{
				if(dx1*dy1<0)
				{
					if(dx2*dy2>0 || dx2*dy1>dx1*dy2) ans++;
				}
				else
				{
					if(dx2*dy2>0 && dx2*dy1>dx1*dy2) ans++;
				}
			}
		}
	}
	for(int i=0;i<n;i++)
	{
		if(b[i].second > b[(i+1)%n].second && b[(i+3)%n].second > b[(i+1)%n].second && b[(i+2)%n].second == b[(i+1)%n].second && b[(i+2)%n].first > b[(i+1)%n].first)
		{
			ans++;
		}
	}
	
	cout<<ans;
	
	return 0;
}