# -*- coding: utf-8 -*-
"""
Created on Fri Jun 21 15:55:33 2019

@author: MalikMehrKhan
"""
n = input("Enter a positive integer  ")
n = int(n)
a = []
b = 1
a.append(b)
while n > b:
    b = b + 1
    a.append(b)
print (sum(a))

