input: Write a function to determine if any two numbers in a given list add up to a given number. If such pair exists, return True, otherwise return False.
  def two_sum(numbers, given):
      for i in range(len(numbers)):
          for j in range(i+1, len(numbers)):
              if numbers[i] + numbers[j] == given:
                  return True
      return False
  assert two_sum([1, 2, 3, 4], 5)==True
  assert two_sum([1, 2, 3, 4], 8)==False
  assert two_sum([1, 2, 3, 4], 4)==True
  assert two_sum([1, 2, 3, 4], 1)==False
  assert two_sum([], 1)==False
output: def two_sum(numbers, given):
      seen = set()
      for num in numbers:
          if given - num in seen:
              return True
          seen.add(num)
      return False