#include <bits/stdc++.h>
#include "trilib.h"
using namespace std;
const int nx=4e4+5;
int n, cnt, qrs, lsz, rsz, tmp[nx];
vector<int> l, r;
deque<int> dl, dr;
void mergesort(int l, int r, int st, vector<int> &x)
{
if (l==r) return;
int md=(l+r)/2, idxl=l, idxr=md+1, idx=l;
mergesort(l, md, st, x);
mergesort(md+1, r, st, x);
while (idx<=r)
{
//cout<<"debug "<<l<<' '<<r<<' '<<idxl<<' '<<idxr<<'\n';
if (idxl>md) tmp[idx++]=x[idxr++];
else if (idxr>r) tmp[idx++]=x[idxl++];
else if (is_clockwise(st, x[idxl], x[idxr])) tmp[idx++]=x[idxl++];
else tmp[idx++]=x[idxr++];
}
for (int i=l; i<=r; i++) x[i]=tmp[i];
}
int main()
{
n=get_n();
for (int i=3; i<=n; i++)
{
if (is_clockwise(1, 2, i)) r.push_back(i);
else l.push_back(i);
}
lsz=l.size();
rsz=r.size();
if (lsz) mergesort(0, lsz-1, 1, l);
if (rsz) mergesort(0, rsz-1, 2, l);
dl.push_back(1);
for (int i=0; i<lsz; i++)
{
while (dl.size()>=2&&!is_clockwise(dl[dl.size()-2], dl.back(), l[i])) dl.pop_back();
dl.push_back(l[i]);
}
dr.push_back(2);
for (int i=0; i<rsz; i++)
{
while (dr.size()>=2&&!is_clockwise(dr[dr.size()-2], dr.back(), r[i])) dr.pop_back();
dr.push_back(r[i]);
}
while (1)
{
if (dl.size()>=2&&dr.size()>=1&&!is_clockwise(dr.back(), dl[0], dl[1]))
{
dl.pop_front();
continue;
}
if (dl.size()>=1&&dr.size()>=2&&!is_clockwise(dr[dr.size()-2], dr.back(), dl[0]))
{
dr.pop_back();
continue;
}
if (dl.size()>=2&&dr.size()>=1&&!is_clockwise(dl[dl.size()-2], dl.back(), dr[0]))
{
dl.pop_back();
continue;
}
if (dl.size()>=1&&dr.size()>=2&&!is_clockwise(dl.back(), dr[0], dr[1]))
{
dr.pop_front();
continue;
}
break;
}
give_answer(dl.size()+dr.size());
return 0;
}
/*
g++ tri3.cpp trilib.h trilib.c -o b
6
1 1
4 3
2 2
1 4
5 1
3 2
*/
