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