import sys
import math

def permutations_count(n, r):
  ret = 0
  for i in range(r):
    ret += (n-i)
  return ret
  
N = int(input())
#print(N)
if N == 1:
  print(1)
  sys.exit()
elif N == 2:
  print(2)
  sys.exit()

r = N - 2

ret = permutations_count(N, r) + 2
print(ret)
