input: Write a python function to search for a key in a list of tuples and return the corresponding value.
  assert search([('a', 2), ('b', 4), ('c', 6), ('d', 7)], 'd') == 7
  assert search([('a', 2), ('b', 4), ('c', 6), ('d', 7)], 'b') == 4
  assert search([('a', 2), ('b', 4), ('c', 6), ('d', 7)], 'z') == "Not found"
  assert search([('x', 10), ('y', 20), ('z', 30)], 'z') == 30
output: def search(lst, key):
      for k, v in lst:
          if k == key:
              return v
      return "Not found"