#include <bits/stdc++.h>
using namespace std;
#define int long long
#define endl '\n'
#define maxn

int n, m;

void solve(){
	cin >> n;
	int t = n / 3;
	int ans = 0;
	if(t >= 2)
		ans += t * (n - 1) - (t - 1) * t / 2;
	else
		ans = n - 1;
	cout << ans<<endl;
}
signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	int T = 1;
	// cin >> T;
	while (T --)
		solve();
}