#include <bits/stdc++.h>
#include "trilib.h"
using namespace std;
const int MAXN = 4e4 + 4;
int n;
int main(void) {
cin.tie(0)->sync_with_stdio(0);
n = get_n();
vector<int> up = {2}, lo, hull;
for (int i = 3; i <= n; i++) {
if (is_clockwise(1, 2, i)) lo.push_back(i);
else up.push_back(i);
}
sort(up.begin(), up.end(), [&](int a, int b) { return !is_clockwise(1, a, b); });
sort(lo.begin(), lo.end(), [&](int a, int b) { return !is_clockwise(1, a, b); });
up.push_back(1);
for (int i: lo) up.push_back(i);
for (int i: up) {
while (hull.size() >= 2 && is_clockwise(hull[hull.size() - 2], hull.back(), i)) hull.pop_back();
hull.push_back(i);
}
int cnt = hull.size();
for (int i = 0; i < cnt; i++) {
while (hull.size() >= 2 && is_clockwise(hull[hull.size() - 2], hull.back(), hull[i])) hull.pop_back();
hull.push_back(hull[i]);
}
give_answer(hull.size() - cnt);
return 0;
}
