input: Write a function to perform a linear search on a list of numbers and return the index of the number if it is present in the list.
  assert linear_search([1, 2, 3, 4, 5], 3) == 2
  assert linear_search([10, 20, 30, 40, 50], 20) == 1
  assert linear_search([5, 15, 25, 35, 45], 10) == -1
output: def linear_search(lst, target):
      for index, value in enumerate(lst):
          if value == target:
              return index
      return -1