#include <bits/stdc++.h>
#include "trilib.h"
using namespace std;
int n, ch[404040], cnt = 0;
vector<int> up, dn;
void solve() {
n = get_n();
for (int i = 3; i <= n; i++) {
if (is_clockwise(1, 2, i)) dn.push_back(i);
else up.push_back(i);
}
sort(up.begin(), up.end(), [](int i, int j){ return is_clockwise(i, 1, j); });
sort(dn.begin(), dn.end(), [](int i, int j){ return !is_clockwise(i, 1, j); });
stack<int> stk;
deque<int> dq;
dq.push_back(1);
dq.push_back(2);
for (int i = 0; i < up.size(); i++) {
int a = dq.back(); dq.pop_back();
while (dq.size() >= 1 && is_clockwise(dq.back(), a, up[i])) {
a = dq.back(); dq.pop_back();
}
dq.push_back(a);
dq.push_back(up[i]);
}
dq.pop_front();
dq.push_back(1);
stk.push(1);
stk.push(2);
for (int i = 0; i < dn.size(); i++) {
int a = stk.top(); stk.pop();
while (stk.size() >= 1 && !is_clockwise(stk.top(), a, dn[i])) {
a = stk.top(); stk.pop();
}
stk.push(a);
stk.push(dn[i]);
}
while (stk.size() > 2) {
int v = stk.top();
stk.pop();
int a = dq.back(); dq.pop_back();
while (dq.size() >= 1 && is_clockwise(dq.back(), a, v)) {
a = dq.back(); dq.pop_back();
}
dq.push_back(a);
dq.push_back(v);
}
for (auto iter = dq.begin(); iter != dq.end(); iter++) ch[*iter] = 1;
sort(up.begin(), up.end(), [](int i, int j){ return !is_clockwise(i, 2, j); });
sort(dn.begin(), dn.end(), [](int i, int j){ return is_clockwise(i, 2, j); });
while (stk.size() > 0) stk.pop();
dq.clear();
dq.push_back(2);
dq.push_back(1);
for (int i = 0; i < up.size(); i++) {
int a = dq.back(); dq.pop_back();
while (dq.size() >= 1 && !is_clockwise(dq.back(), a, up[i])) {
a = dq.back(); dq.pop_back();
}
dq.push_back(a);
dq.push_back(up[i]);
}
dq.pop_front();
dq.push_back(2);
stk.push(2);
stk.push(1);
for (int i = 0; i < dn.size(); i++) {
int a = stk.top(); stk.pop();
while (stk.size() >= 1 && is_clockwise(stk.top(), a, dn[i])) {
a = stk.top(); stk.pop();
}
stk.push(a);
stk.push(dn[i]);
}
while (stk.size() > 2) {
int v = stk.top();
stk.pop();
int a = dq.back(); dq.pop_back();
while (dq.size() >= 1 && !is_clockwise(dq.back(), a, v)) {
a = dq.back(); dq.pop_back();
}
dq.push_back(a);
dq.push_back(v);
}
for (auto iter = dq.begin(); iter != dq.end(); iter++) cnt += ch[*iter];
give_answer(cnt);
}
int main() {
solve();
}
