#include<bits/stdc++.h>
using namespace std;
#define db double
#define ll long long 
#define N 2010<<1
int n;
int nxt[N];
bool b[N];
const db eps=1e-10;
struct node{
    db x,y;
    node(){}
    node(db _x,db _y):x(_x),y(_y){}
    node operator-(const node &t)const {return {x-t.x,y-t.y};}
    bool operator==(const node &t) const {return x==t.x&&y==t.y;}
}e[N];
db cross(node a,node b) {
    return a.x*b.y-b.x*a.y;
}
int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);cout.tie(0);
    cin>>n;
    for(int i=1;i<=n;i++) cin>>e[i].x>>e[i].y;
    for(int i=n+1;i<=2*n;i++) e[i]=e[i-n];
    nxt[2*n]=2*n;
    for(int i=2*n-1;i>=1;i--) {
        if(e[i].y==e[i+1].y) nxt[i]=nxt[i+1];
    }
    int ans=0;
    for(int i=n;i<2*n;i++) {
        int s=0;
        for(int j=1;j<=n;j++) {
            node c=e[j],d=e[j+1];
            if(c==e[i] || d==e[i]) continue;
            if(c.x==e[i].x || d.x==e[i].x) continue;
            if(c.x>d.x) swap(c,d);
            if(cross(d-e[i],c-e[i])<=0) continue;
            if(e[i].x>min(c.x,d.x) && e[i].x<max(c.x,d.x)) s++;
        }
        for(int j=1;j<=n;j++) {
            if(j==i) continue;
            if(e[i].x!=e[j].x) continue;
            if(e[j].y<=e[i].y) continue;
            node c=e[(j-1==0?n:j-1)],d=e[j+1];
            db g=cross(c-e[i],e[j]-e[i]),h=cross(d-e[i],e[j]-e[i]);
            if(fabs(g*h)<eps) {
                if(c.x==e[i].x && d.x==e[i].x) continue;
                s++;
            }
            else {
                if(g*h>eps) {
                    s+=2;
                }
                else {
                    s++;
                }
            }
        }
        // cout<<i<<" "<<s<<endl;
        if(s&1) b[i]=1;
    }
    // for(int i=n;i<=2*n;i++) cout<<b[i]<<" ";cout<<endl;
    for(int i=n;i<2*n;i++) {
        if(!b[i]) continue;
        if(e[i].y<e[i-1].y && e[i].y<e[i+1].y) ans++;//,cout<<(i-n==0?n:i-n)<<endl;
    }
    // cout<<ans<<endl;
    for(int i=n;i<2*n;i++) {
        if(nxt[i]+1>2*n) continue;
        if(nxt[i]==i) continue;
        if(e[i].y<e[i-1].y && e[i].y<e[nxt[i]+1].y) ans++;
    }
    cout<<ans<<endl;
    return 0;
}