input: Write a python function to determine the number of negative elements in a list.
  assert count_negatives([-1,-2,3,-4,-5]) == 4
  assert count_negatives([1,2,3]) == 0
  assert count_negatives([1,2,-3,-10,20]) == 2
output: def count_negatives(lst):
      return sum(1 for x in lst if x < 0)