input: Write a python function to perform linear search on a list.
  assert linear_search([1, 2, 3, 4, 5], 3) == 2
  assert linear_search([10, 20, 30, 40, 50], 10) == 0
  assert linear_search([15, 25, 35, 45, 55], 55) == 4
  assert linear_search([5, 6, 7, 8, 9], 1) == -1
output: def linear_search(lst, target):
      for index, value in enumerate(lst):
          if value == target:
              return index
      return -1