#include<bits/stdc++.h>
using namespace std;
struct node{
	int x;
	int y;
}no[1000050];
//vector<node>v;
vector<int>tem;
bool check(int l,int r,int mid){
	int y1=no[mid].y-no[l].y;
	int x1=no[mid].x-no[l].x;
	int x2=no[r].x-no[l].x;
	int y2=no[r].y-no[l].y;
	return ((x1*y2-x2*y1)>0);
}
void solve(){
	int n;
	scanf("%d",&n);
	int st=0;
	for(int i=0;i<=n-1;i++){
		scanf("%d %d",&no[i].x,&no[i].y);
	}
	for(int i=0;i<n-1;i++){
		if(no[i+1].y==no[i].y){
			st++;
		}else{
			break;
		}
	}
	int l=st,mid,r;
	mid=(l+1)%n;r=(mid+1)%n;
	int cnt=0;
	while(1){
		tem.clear();
		while(no[r].y==no[mid].y){
			tem.push_back(mid);
			mid++;
			mid%=n;
			r=(mid+1)%n;	
		}
		int now=0;
		r=(mid+1)%n;
		for(auto mi:tem){
			if(no[l].y>no[mi].y&&no[r].y>no[mi].y){
				if(check(l,r,mi)){
					now++;
				}
			}
		}
		if(no[l].y>no[mid].y&&no[r].y>no[mid].y){
			if(check(l,r,mid)){
				now++;
			}
		}
		cnt+=(now>0);
		l=mid;mid++;
		mid%=n;
		r=(mid+1)%n;
		if(l==st){
			break;
		}
	}
	printf("%d",cnt);
}

int main(){
	int t=1;
//	scanf("%d",&t);
	while(t--){
		solve();
		if(t) printf("\n");
	}
	return 0;
}
/*
10
1 0
2 0
3 0
2 1
3 2
2 2
1 2
0 2
1 1
0 0
//1


6
0 0
2 0
1 1
4 1
5 0
3 4
//2

16
0 0
60 0
60 70
0 70
0 20
40 20
40 50
20 50
20 40
30 40
30 30
10 30
10 60
50 60
50 10
0 10
//2

16
2 0
3 0
3 1
5 1
5 6
0 6
0 4
1 4
1 3
2 3
2 5
3 5
3 3
4 3
4 2
2 2
//2
*/