input: Write a function to find the nth smallest number in a list.
  assert nth_smallest([2, 3, 5, 7, 11], 2) == 3
  assert nth_smallest([10, 20, 30, 40, 50], 4) == 40
  assert nth_smallest([1, 1, 1, 1, 1], 1) == 1
output: def nth_smallest(lst, n):
      lst.sort()
      return lst[n-1]