SPIRAL TS10 PTNK 2018-2019

#define task "SPIRAL"

#include <iostream>
#include <cstdio>

using namespace std;

using ll = long long;
using ld = long double;

constexpr int N = 1e5 + 5;
int n;

void Read()
{
    cin >> n;
}

void O1_Solution()
{
    // Ta có 1 * 1 + 2 * 2 + ... + n * n = n * (n + 1) * (2n + 1) / 6
    // Đáp án là   n + [n + (n-1) * 2] + [n + (n-1) * 2 + (n-2) * 2] + ... + [n + (n-1) * 2 + (n-2) * 2 + ... + 1 * 2]
    //           = n * n + 2 * [(n-1) * (n-1) + (n-2) * (n-2) + ... + 1 * 1]
    //           = n * n + 2 * (n - 1) * [2 * (n - 1) + 1] * [(n - 1) + 1] / 6
    //           = n * n + (n - 1) * (2 * n - 1) * n / 3

    cout << 1ll * n * n + 1ll * (n - 1) * (2 * n - 1) * n / 3;
}

void On_Solution()
{
    ll a = n, ans = n;

    for (int i = 2; i <= n; ++i)
    {
        a = a + (n - i + 1) * 2;
        ans = ans + a;
    }

    cout << ans;
}

int32_t main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    if (fopen(task ".INP", "r"))
    {
        freopen(task ".INP", "r", stdin);
        freopen(task ".OUT", "w", stdout);
    }

    Read();
    // O1_Solution();
    On_Solution();
}