output_description: Print the area of the triangle ABC.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      |AB| |BC| |CA|
attempt: from collections import defaultdict, Counter
  from itertools import product, groupby, count, permutations, combinations
  from math import pi, sqrt
  from collections import deque
  from bisect import bisect, bisect_left, bisect_right
  from string import ascii_lowercase
  from functools import lru_cache
  import sys
  
  sys.setrecursionlimit(10000)
  INF = float("inf")
  YES, Yes, yes, NO, No, no = "YES", "Yes", "yes", "NO", "No", "no"
  dy4, dx4 = [0, 1, 0, -1], [1, 0, -1, 0]
  dy8, dx8 = [0, -1, 0, 1, 1, -1, -1, 1], [1, 0, -1, 0, 1, 1, -1, -1]
  
  
  def inside(y, x, H, W):
      return 0 <= y < H and 0 <= x < W
  
  
  def ceil(a, b):
      return (a + b - 1) // b
  
  
  # aとbの最大公約数
  def gcd(a, b):
      if b == 0:
          return a
      return gcd(b, a % b)
  
  
  # aとbの最小公倍数
  def lcm(a, b):
      g = gcd(a, b)
      return a / g * b
  
  
  def main():
      AB, BC, CA = map(int, input().split())
      print(AB * BC // 2)
  
  
  if __name__ == "__main__":
      main()
  
problem_description: Statement
  
  There is a right triangle ABC with ∠ABC=90°.
  
  Given the lengths of the three sides, |AB|,|BC| and |CA|, find the area of the
  right triangle ABC.
  
  It is guaranteed that the area of the triangle ABC is an integer.
samples: [{"input": "3 4 5", "output": "6\n    \n\n![tri](https://img.atcoder.jp/ghi/44c1d4cbdf4677ce3b08ca70b5ce98ea.png)\n\nThis triangle has an area of 6.\n\n* * *"}, {"input": "5 12 13", "output": "30\n    \n\nThis triangle has an area of 30.\n\n* * *"}, {"input": "45 28 53", "output": "630\n    \n\nThis triangle has an area of 630."}]