#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define FOR(i,a,b) for (int i = (a); i < (b); i++)
using namespace std;
using namespace __gnu_pbds;
using ll=long long;
using ld=long double;
using vi=vector<int>;
template<class T> using oset =tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update> ;
//上
struct pt{
    ll x,y;
    pt():x(0),y(0){}
    pt(ll x,ll y): x(x),y(y){}
    pt operator+(pt p){return pt(x+p.x,y+p.y);}
    pt operator-(pt p){return pt(x-p.x,y-p.y);}
    ll cross(pt p){return (*this).x*p.y-(*this).y*p.x;}
};
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    ll n;
    cin >> n;
    vector<pt> v(n);
    FOR(i,0,n) cin >> v[i].x >> v[i].y;
    int index = 0;
    for(int i = 0; i< n; i++){
        if(v[(i+1)%n].y > v[i].y){
            index = i;
            break;
        }
    }
    int p1 = (index+1)%n;
    int ans = 0;
    int need = 0;
    for(int i = 0; i < n; i++){
        if(v[(p1+1)%n].y < v[p1].y){
            need = 1;
        }
        if(need && v[(p1+1)%n].y > v[p1].y && (v[(p1+1)%n]-v[p1]).cross(v[(p1+n-1)%n]-v[(p1)%n]) >= 0){
            ans++;
            need = 0;
        }
        p1++;
        p1%=n;
    }
    cout << ans <<'\n';

}