/// In The Name Of God
#include <bits/stdc++.h>
#include "trilib.h"
#define f first
#define s second
#define pb push_back
#define pp pop_back
#define mp make_pair
#define sz(x) (int)x.size()
#define sqr(x) ((x) * 1ll * (x))
#define all(x) x.begin(), x.end()
#define rep(i, l, r) for (int i = (l); i <= (r); i++)
#define per(i, l, r) for (int i = (l); i >= (r); i--)
#define Kazakhstan ios_base :: sync_with_stdio(0), cin.tie(0), cout.tie(0);
#define nl '\n'
#define ioi exit(0);
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
const int N = (int)5e5 + 7;
const int inf = (int)1e9 + 7;
const int mod = (int)1e9 + 7;
const ll linf = (ll)1e18 + 7;
const int dx[] = {-1, 0, 1, 0, 1, -1, -1, 1};
const int dy[] = {0, 1, 0, -1, 1, -1, 1, -1};
using namespace std;
int n;
bool check(int i, int j) {
set <int> st;
for (int k = 1; k <= n; k++) {
if (k == i || k == j) continue;
st.insert(is_clockwise(i, j, k));
if (sz(st) > 1) return 0;
}
return 1;
}
int main() {
#ifdef IOI2018
freopen ("in.txt", "r", stdin);
#endif
n = get_n();
set <int> st;
rep(i, 1, n) {
int cnt = 0;
rep(j, i + 1, n) {
if (check(i, j)) {
st.insert(i), st.insert(j);
++cnt;
if (cnt == 2) break;
}
}
}
give_answer(sz(st));
ioi
}
