input: Write a python function to compute the nth Fibonacci number recursively.
  assert fib(1) == 0
  assert fib(2) == 1
  assert fib(3) == 1
  assert fib(10) == 34
output: def fib(n):
      if n == 1:
          return 0
      elif n == 2:
          return 1
      else:
          return fib(n-1) + fib(n-2)