等差数列

#include<bits/stdc++.h>
using namespace std;
int main()
    {
    int n;
    while(cin>>n)
        {
        if(n<=0)cout<<"-1"<<endl;
        else 
            {
             cout<<(2+(n-1)*3+2)*n/2<<endl;   
        }
    }
    return 0;
}