#include<iostream>
using namespace std;
#define MAX 10010

int areanum[MAX];

int getarean(int n)
{
    return (areanum[n]!=0)? areanum[n] : getarean(n-1)+n;
}

int main()
{
    areanum[1] = 2;
    int n;
    while(cin >> n)
    {
        cout << getarean(n) <<endl;
    }
    return 0;
}