code: def ncr(n, r):
      
      r = min(r, n - r)
      numer = reduce(op.mul, range(n, n - r, -1), 1)
      denom = reduce(op.mul, range(1, r + 1), 1)
      return numer // denom
docs: Calculate n choose r.
  
      :param n: n
      :type n : int
      :param r: r
      :type r :int
      :return: n choose r as int