input: Write a python function to generate the nth Fibonacci number using recursion.
  assert fib(0) == 0
  assert fib(1) == 1
  assert fib(5) == 5
  assert fib(10) == 55
output: def fib(n):
      if n <= 1:
          return n
      else:
          return fib(n-1) + fib(n-2)