#include<stdio.h>
#include"trilib.h"
#include<vector>
#include<algorithm>
using namespace std;
vector<int>L, R, LCV, RCV;
int ccw(int a, int b, int c) {
return is_clockwise(a, b, c); }
int t;
bool sort_angle(int a, int b) {
if (a == 1)return 1;
if (b == 1)return 0;
return t^ccw(1, a, b);
}
void get_CV(vector<int>&a, vector<int>&res, int tt) {
t = tt;
sort(a.begin(), a.end(), sort_angle);
for (int i = 0; i < a.size(); i++) {
while (res.size() >= 2 && (!t)^ccw(res[res.size() - 2], res[res.size() - 1], a[i]))res.pop_back();
res.push_back(a[i]);
}
}
int LV[41414],RV[41414];
void VALIDCK(vector<int>&LCV, vector<int>&RCV, int LV[],int t) {
int i, pj, nj;
if (RCV.size() == 2)return;
pj = 2;
for (i = 1; i < LCV.size(); i++) {
int ni = (i + 1) % LCV.size();
while (pj != 0 && (!t)^ccw(RCV[pj], LCV[i], LCV[0]) && t^ccw(RCV[pj], LCV[ni], LCV[i])) {
pj = (pj + 1) % RCV.size();
}
if (pj==0 || t^ccw(RCV[pj], LCV[i], LCV[0]))break;
LV[i] = 0;
}
pj = RCV.size()-1;
for (i = 0; i != 1; i = (i + LCV.size() - 1) % LCV.size()) {
int ni = (i + LCV.size() - 1) % LCV.size();
while (pj!=1 && t^ccw(RCV[pj],LCV[i],LCV[1]) && (!t)^ccw(RCV[pj], LCV[ni], LCV[i])) {
pj = (pj + RCV.size() - 1) % RCV.size();
}
if (pj == 1 || (!t)^ccw(RCV[pj], LCV[i], LCV[1]))break;
LV[i] = 0;
}
}
int main() { int n = get_n(), i, j;
L.push_back(1), L.push_back(2);
R.push_back(1), R.push_back(2);
for (i = 3; i <= n; i++) {
if (ccw(1, 2, i))L.push_back(i);
else R.push_back(i);
}
get_CV(L, LCV,0), get_CV(R, RCV, 1);
for (i = 0; i < LCV.size(); i++)LV[i] = 1;
for (i = 0; i < RCV.size(); i++)RV[i] = 1;
VALIDCK(LCV, RCV, LV,1);
VALIDCK(RCV, LCV, RV,0);
int ans = 0;
for (i = 0; i < LCV.size(); i++)ans += LV[i];
for (i = 0; i < RCV.size(); i++)ans += RV[i];
ans -= LV[0]; ans -= LV[1];
give_answer(ans);
return 0;
}
