output_description: Print the necessary number of candies in total.
  
  * * *
input_description: The input is given from Standard Input in the following format:
  
      
      
      N
attempt: from itertools import combinations, product
  
  
  def gcd(a, b):
      if a < b:
          a, b = b, a
      while b != 0:
          a, b = b, a % b
      return a
  
  
  def lcm(a, b):
      return a * b // gcd(a, b)
  
  
  def get(fmt):
      ps = input().split()
      r = []
      for p, t in zip(ps, fmt):
          if t == "i":
              r.append(int(p))
          if t == "f":
              r.append(float(p))
          if t == "s":
              r.append(p)
      if len(r) == 1:
          r = r[0]
      return r
  
  
  def put(*args, **kwargs):
      print(*args, **kwargs)
  
  
  def rep(n, f, *args, **kwargs):
      return [f(*args, **kwargs) for _ in range(n)]
  
  
  def rep_im(n, v):
      return rep(n, lambda: v)
  
  
  YES_NO = ["NO", "YES"]
  
  
  N = get("i")
  
  put(N * (N - 1) // 2)
  
problem_description: Statement
  
  There are N children in AtCoder Kindergarten. Mr. Evi will arrange the
  children in a line, then give 1 candy to the first child in the line, 2
  candies to the second child, ..., N candies to the N-th child. How many
  candies will be necessary in total?
samples: [{"input": "3", "output": "6\n    \n\nThe answer is 1+2+3=6.\n\n* * *"}, {"input": "10", "output": "55\n    \n\nThe sum of the integers from 1 to 10 is 55.\n\n* * *"}, {"input": "1", "output": "1\n    \n\nOnly one child. The answer is 1 in this case."}]