SPIRAL C++

#include <bits/stdc++.h>
using namespace std;
long long n;
void giai()
{
    long long res=n;
    long long  a[n+100];
    a[1]=n;
    long long kc = n*2-2;
    for(int i=2;i<=n;i++)
    {
        a[i]=a[i-1]+kc;
        res+=a[i];
        kc-=2;
    }
    cout<<res;
}
int main()
{
    ios_base::sync_with_stdio(0);
    cout.tie(0); cin.tie(0);
    freopen("spiral.inp","r",stdin);
    freopen("spiral.out","w",stdout);
    cin>>n;
    giai();
}