#include <stdio.h>
#include "trilib.h"
#include <vector>
#include <algorithm>
#include <queue>
#define pii pair<int,int>
using namespace std;
int piv = 1;
bool cmp(int a, int b) {
return is_clockwise(piv,b,a);
}
void make_hull(vector<int> &h,vector<int> &v) {
int i;
for (i = 0; i < 2 && i < v.size(); ++i) h.push_back(v[i]);
for (; i < v.size(); ++i) {
while (h.size() > 1 && is_clockwise(h[h.size() - 2], h.back(), v[i]))
h.pop_back();
h.push_back(v[i]);
}
}
int main() {
int n = get_n();//[1,n]
vector<int > u, d;
vector<int> hu, hd,ans;
for (int i = 3; i <= n; ++i) {
if (is_clockwise(1, 2, i)) d.push_back(i);
else u.push_back(i);
}
stable_sort(u.begin(), u.end(), cmp);
stable_sort(d.begin(), d.end(), cmp);
make_hull(hu, u); make_hull(hd, d);
//for (auto p : hu) printf("u:%d\n", p);
//for (auto p : hd) printf("d:%d\n", p);
hu.push_back(1);
for (auto p : hd) hu.push_back(p);
hu.push_back(2);
make_hull(ans,hu);
//for (auto p : ans) printf("a1:%d\n", p);
bool flag = 1;
int pad = 0;
while (flag) {
flag = 0;
while (ans.size() - pad > 2 && is_clockwise(ans[ans.size() - 2], ans.back(), ans[pad]))
ans.pop_back(),flag=1;
while (ans.size() - pad > 2 && is_clockwise(ans.back(), ans[pad], ans[pad+1]))
++pad, flag = 1;
}
give_answer(ans.size()-pad);
//for(int i=pad;i<ans.size();++i) printf("a2:%d\n", ans[i]);
return 0;
}
