input: Write a python function to find the index of a given number in a list.
  assert find_number([2, 4, 9, 8, 10], 8) == 3
  assert find_number([2, 4, 9, 8, 10], 5) == -1
  assert find_number([1, 2, 3, 4, 5], 3) == 2
  assert find_number([], 3) == -1
  assert find_number([7, 7, 7, 7], 7) == 0
output: def find_number(lst, num):
      try:
          return lst.index(num)
      except ValueError:
          return -1