input: Write a function to check if any two numbers from a given list sum up to a specified target number. If such a pair exists, return True; otherwise, return False.
  assert check_sum([1, 2, 3, 4], 5)==True
  assert check_sum([1, 2, 3, 4], 8)==False
  assert check_sum([10, 20, 30], 50)==True
  assert check_sum([5, 10, 15], 7)==False
output: def check_sum(nums, target):
      seen = set()
      for num in nums:
          if target - num in seen:
              return True
          seen.add(num)
      return False