#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 comp(int x, int y) {
return is_clockwise(1, 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);
}
sort(all(a), comp);
sort(all(b), comp);
vi c = {1};
for (int i: a)
c.push_back(i);
c.push_back(2);
for (int i: b)
c.push_back(i);
vi st;
for (int i: c) {
while (st.size() > 1 && is_clockwise(st[(int)st.size() - 2], st.back(), i))
st.pop_back();
st.push_back(i);
}
for (int i: c) {
while (st.size() > 1 && is_clockwise(st[(int)st.size() - 2], st.back(), i))
st.pop_back();
st.push_back(i);
}
vi cnt(n, 0);
int ans = 0;
for (int i: st) {
++cnt[i];
if (cnt[i] == 2)
++ans;
}
give_answer(ans);
}
