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: #!/usr/bin/env python3
  from collections import defaultdict
  from collections import deque
  from heapq import heappush, heappop
  import sys
  import math
  import bisect
  import random
  import itertools
  
  sys.setrecursionlimit(10**5)
  stdin = sys.stdin
  bisect_left = bisect.bisect_left
  bisect_right = bisect.bisect_right
  
  
  def LI():
      return list(map(int, stdin.readline().split()))
  
  
  def LF():
      return list(map(float, stdin.readline().split()))
  
  
  def LI_():
      return list(map(lambda x: int(x) - 1, stdin.readline().split()))
  
  
  def II():
      return int(stdin.readline())
  
  
  def IF():
      return float(stdin.readline())
  
  
  def LS():
      return list(map(list, stdin.readline().split()))
  
  
  def S():
      return list(stdin.readline().rstrip())
  
  
  def IR(n):
      return [II() for _ in range(n)]
  
  
  def LIR(n):
      return [LI() for _ in range(n)]
  
  
  def FR(n):
      return [IF() for _ in range(n)]
  
  
  def LFR(n):
      return [LI() for _ in range(n)]
  
  
  def LIR_(n):
      return [LI_() for _ in range(n)]
  
  
  def SR(n):
      return [S() for _ in range(n)]
  
  
  def LSR(n):
      return [LS() for _ in range(n)]
  
  
  mod = 1000000007
  inf = float("INF")
  
  
  # A
  def A():
      a, b, h = IR(3)
      print((a + b) * h / 2)
      return
  
  
  # B
  def B():
      return
  
  
  # C
  def C():
      return
  
  
  # D
  def D():
      return
  
  
  # E
  def E():
      return
  
  
  # F
  def F():
      return
  
  
  # G
  def G():
      return
  
  
  # H
  def H():
      return
  
  
  # Solve
  if __name__ == "__main__":
      A()
  
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."}]