#include "trilib.h"
//#include "trilib.c"
#include <bits/extc++.h>
using namespace std;
using namespace __gnu_pbds;
#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define all(a) a.begin(), a.end()
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using vi = vector<int>;
using oset = tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>;
bool is_not_inside(vi &a, int i) {
if (a.size() < 3)
return true;
for (int j = 0; j < (int)a.size(); ++j) {
if (is_clockwise(a[j], a[(j + 1) % (int)a.size()], i))
return true;
}
return false;
}
int O;
bool comp(int x, int y) {
return is_clockwise(O, y, x);
}
int main() {
int n = get_n();
vi a, b;
for (int i = 3; i <= n; ++i) {
if (is_clockwise(1, 2, i))
a.push_back(i);
else
b.push_back(i);
}
O = 2;
sort(all(a), comp);
O = 1;
sort(all(b), comp);
vi st1 = {2, 1};
for (int i: a) {
while (is_clockwise(st1[(int)st1.size() - 2], st1.back(), i))
st1.pop_back();
st1.push_back(i);
}
vi st2 = {1, 2};
for (int i: b) {
while (is_clockwise(st2[(int)st2.size() - 2], st2.back(), i))
st2.pop_back();
st2.push_back(i);
}
vi c = {1};
for (int i = 2; i < (int)st1.size(); ++i)
c.push_back(st1[i]);
c.push_back(2);
for (int i = 2; i < (int)st2.size(); ++i)
c.push_back(st2[i]);
int ans = (int)c.size();
for (int i = 0; i < (int)c.size(); ++i) {
if (is_clockwise(c[i], c[(i + 1) % (int)c.size()], c[(i + 2) % (int)c.size()]))
--ans;
}
give_answer(ans);
}
