input: Write a function to check if any two numbers in a list add up to a given value.
  assert two_sum([1, 2, 3, 4, 5], 9) == False
  assert two_sum([1, 2, 3, 4, 5], 8) == True
  assert two_sum([1, 2, 3, 4, 5], 5) == True
  assert two_sum([1, 2, 3, 4, 5], 10) == False
output: def two_sum(lst, target):
      seen = set()
      for num in lst:
          if target - num in seen:
              return True
          seen.add(num)
      return False