input: Write a function to compute the nth Fibonacci number recursively.
  assert fibonacci(1) == 1
  assert fibonacci(2) == 1
  assert fibonacci(5) == 5
  assert fibonacci(10) == 55
output: def fibonacci(n):
      if n <= 0:
          raise ValueError("n must be a positive integer")
      elif n == 1 or n == 2:
          return 1
      else:
          return fibonacci(n - 1) + fibonacci(n - 2)