#include "trilib.h"
#include <iostream>
#include <vector>
#include <random>
#include <algorithm>
#include <cassert>
#include <cstring>
using namespace std;
/*int get_n();
bool is_clockwise(int a, int b, int c);
void give_answer(int s);*/
bool upper_cmp(int i, int j) {
if (i == j) return false;
return !is_clockwise(2, i + 1, j + 1);
}
bool lower_cmp(int i, int j) {
if (i == j) return false;
return is_clockwise(2, i + 1, j + 1);
}
vector<int> hull_upper(vector<int> sorted_points) {
vector<int> hull;
for (int i : sorted_points) {
while (hull.size() > 1 && is_clockwise(hull[hull.size() - 2] + 1, hull.back() + 1, i + 1)) hull.pop_back();
hull.push_back(i);
}
return hull;
}
vector<int> hull_lower(vector<int> sorted_points) {
vector<int> hull;
for (int i : sorted_points) {
while (hull.size() > 1 && !is_clockwise(hull[hull.size() - 2] + 1, hull.back() + 1, i + 1)) hull.pop_back();
hull.push_back(i);
}
return hull;
}
int32_t main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n = get_n();
vector<int> upper, lower;
for (int i = 2; i < n; ++i) {
if (is_clockwise(1, 2, i + 1)) {
lower.push_back(i);
} else {
upper.push_back(i);
}
}
sort(upper.begin(), upper.end(), upper_cmp);
upper.insert(upper.begin(), 1);
upper.push_back(0);
sort(lower.begin(), lower.end(), lower_cmp);
lower.insert(lower.begin(), 1);
lower.push_back(0);
vector<int> upper_hull = hull_upper(upper);
vector<int> lower_hull = hull_lower(lower);
if (upper_hull.size() == 2) {
give_answer(lower_hull.size());
return 0;
}
reverse(lower_hull.begin(), lower_hull.end());
for (int i : lower_hull) {
upper_hull.push_back(i);
}
cerr << "candidates: ";
for (int i : upper_hull) {
cerr << i << ' ';
}
cerr << endl;
vector<int> hull;
vector<bool> in(n, true);
for (int s = 0; s < upper_hull.size() * 2; ++s) {
int i = upper_hull[(1 + s) % upper_hull.size()];
if (!hull.empty() && i == upper_hull[(s) % upper_hull.size()]) continue;
while (hull.size() > 1 && is_clockwise(hull[hull.size() - 2] + 1, hull.back() + 1, i + 1)) {
in[hull.back()] = false;
hull.pop_back();
}
hull.push_back(i);
}
int ans = 0;
for (int i : hull) {
if (in[i]) {
++ans;
in[i] = false;
}
}
give_answer(ans);
return 0;
}
