output_description: Print the area of the given trapezoid. It is guaranteed that the area is an
  integer.
  
  * * *
input_description: The input is given from Standard Input in the following format:
  
      
      
      a
      b
      h
attempt: from functools import reduce
  
  
  def main():
      # 初期値用:十分大きい数(100億)
      # 1e10
  
      # 初期値用:十分小さい数(-100億)
      # -1e10
  
      # 1文字のみを読み込み
      # s = input().rstrip()
  
      # スペース区切りで標準入力を配列として読み込み
      # s = input().rstrip().split(' ')
  
      # 1文字ずつ標準入力を文字列として読み込み
      # s = list(input().rstrip())
  
      # 位置文字ずつ標準入力を数値配列として読み込み
      # X = [int(_) for _ in input().split()]
  
      N = int(input().rstrip())
      K = int(input().rstrip())
      X = int(input().rstrip())
      print(int((N + K) * X / 2))
  
  
  if __name__ == "__main__":
      main()
  
problem_description: Statement
  
  You are given a trapezoid. The lengths of its upper base, lower base, and
  height are a, b, and h, respectively.
  
  ![](https://atcoder.jp/img/arc061/1158e37155d46a42e90f31566478e6da.png)
  
  An example of a trapezoid
  
  Find the area of this trapezoid.
samples: [{"input": "3\n    4\n    2", "output": "7\n    \n\nWhen the lengths of the upper base, lower base, and height are 3, 4, and 2,\nrespectively, the area of the trapezoid is (3+4)\u00d72/2 = 7.\n\n* * *"}, {"input": "4\n    4\n    4", "output": "16\n    \n\nIn this case, a parallelogram is given, which is also a trapezoid."}]