input: Write a function to perform a linear search.
  assert linear_search([1, 2, 3, 4, 5], 3) == 2
  assert linear_search([10, 20, 30, 40, 50], 40) == 3
  assert linear_search([7, 14, 21, 28, 35], 5) == -1
output: def linear_search(arr, target):
      for i in range(len(arr)):
          if arr[i] == target:
              return i
      return -1