#include "trilib.h"
#include<bits/stdc++.h>
#define eb emplace_back
#define iter(a) a.begin(), a.end()
#define lsort(a) sort(iter(a))
#define uni(a) a.resize(unique(iter(a)) - a.begin())
#define printv(a, b) {\
for(auto pv : a) b << pv << " ";\
b << "\n";\
}
using namespace std;
typedef long long ll;
void waassert(bool b){
if(b) return;
cout << "OAO\n";
exit(0);
}
int n;
vector<int> hull;
int query(int a, int b, int c){
//waassert(a != b && b != c && c != a);
return is_clockwise(a, b, c);
}
void add(int id){
//cerr << "test " << id << "\n";
int m = hull.size();
bool lst = query(id, hull[m - 1], hull[0]);
vector<bool> del(m);
int pos = -1;
for(int i = 0; i < m; i++){
bool now = query(id, hull[i], hull[(i + 1) % m]);
//cerr << i << " " << hull[i] << " " << hull[(i + 1) % m] << " " << lst << " " << now << "\n";
if(now && lst) del[i] = true;
else if(now && pos == -1) pos = i;
lst = now;
}
//printv(del, cerr);
vector<int> tmp;
tmp.swap(hull);
for(int i = 0; i < m; i++){
if(!del[i]) hull.eb(tmp[i]);
if(i == pos) hull.eb(id);
}
//cerr << "ok ";
//printv(hull, cerr);
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
n = get_n();
hull.eb(1);
hull.eb(2);
hull.eb(3);
for(int i = 4; i <= n; i++) add(i);
give_answer(hull.size());
return 0;
}
