#include <bits/stdc++.h>
#include "trilib.h"
#define int long long
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
using namespace std;
signed main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int n=get_n();
int root=1;
int x1,x2;
vector<int> a;
vector<int> b;
a.push_back(2);
b.push_back(2);
for (int i = 3; i <= n; i++)
{
if(i==root) continue;
if(is_clockwise(1,2,i)) a.push_back(i);
else b.push_back(i);
}
sort(all(a), [root](int _a, int _b){ return is_clockwise(root,_a,_b); });
sort(all(b), [root](int _a, int _b){ return !is_clockwise(root,_a,_b); });
vector<int> convA;
convA.push_back(root);
convA.push_back(a[0]);
for (int i = 1; i < sz(a); i++)
{
while(true){
x2=convA.back();
convA.pop_back();
x1=convA.back();
if(is_clockwise(x1,x2,a[i])){
convA.push_back(x2);
break;
}
}
convA.push_back(a[i]);
}
reverse(all(convA));
convA.pop_back();
reverse(all(convA));
convA.push_back(root);
vector<int> convB;
convB.push_back(root);
convB.push_back(b[0]);
for (int i = 1; i < sz(b); i++)
{
while(true){
x2=convB.back();
convB.pop_back();
x1=convB.back();
if(!is_clockwise(x1,x2,b[i])){
convB.push_back(x2);
break;
}
}
convB.push_back(b[i]);
}
reverse(all(convB));
convB.pop_back();
reverse(all(convB));
convB.push_back(root);
reverse(all(convB));
vector<int> convAB,conv;
for (int i = 1; i < sz(convA); i++)
{
if(sz(convAB)>0&&convAB.back()==convA[i]) continue;
convAB.push_back(convA[i]);
}
for (int i = 1; i < sz(convB); i++)
{
if(sz(convAB)>0&&convAB.back()==convB[i]) continue;
convAB.push_back(convB[i]);
}
int sAB=sz(convAB);
for (int i = 0; i < sAB; i++)
{
convAB.push_back(convAB[i]);
}
conv.push_back(convAB[0]);
conv.push_back(convAB[1]);
for (int i = 2; i < sz(convAB); i++)
{
while(true){
x2=conv.back();
conv.pop_back();
x1=conv.back();
if(is_clockwise(x1,x2,convAB[i])){
conv.push_back(x2);
break;
}
}
conv.push_back(convAB[i]);
}
vector<int> cnt(n+1);
int sm=0;
for (int i = 0; i < sz(conv); i++)
{
cnt[conv[i]]++;
if(cnt[conv[i]]==2) sm++;
}
give_answer(sm);
return 0;
}
