input: Write a function to count the number of pairs in a list of numbers that sum up to a given value x. If the list is empty return 0.
  assert count_pairs([1, 2, 3, 4, 5, 6], 10)==2
  assert count_pairs([1, 2, 3, 4, 5], 15)==0
  assert count_pairs([1, 9, 2, 8, 3, 7, 4, 6], 10)==4
  assert count_pairs([], 10)==0
  assert count_pairs([5, 5, 5, 5, 5], 10)==2
output: def count_pairs(nums, x):
      count = 0
      seen = set()
      for num in nums:
          if x - num in seen:
              count += 1
          seen.add(num)
      return count