# sumprod 
# Fig. 15.1 
# input: 10

import random

def f(n):
  if n < 3:
    return n-1
  else: 
    k = random.randint(1,n-1)
    return k*(n-k) + f(k) + f(n-k)

n = int(input("n=? "))
print(f(n))