input: Write a function to perform linear search.
  assert linear_search([7, 4, 3, 11, 9], 11) == 3
  assert linear_search([7, 4, 3, 11, 9], 4) == 1
  assert linear_search([7, 4, 3, 11, 9], 9) == 4
  assert linear_search([7, 4, 3, 11, 9], 1) == -1
output: def linear_search(arr, target):
      for index, value in enumerate(arr):
          if value == target:
              return index
      return -1