input: Write a function to generate Fibonacci numbers using a recursive approach. The function should take an integer `n` as input and return the nth Fibonacci number.
  assert fibonacci(1) == 1
  assert fibonacci(2) == 1
  assert fibonacci(10) == 55
  assert fibonacci(15) == 610
  assert fibonacci(20) == 6765
output: def fibonacci(n):
      if n == 1 or n == 2:
          return 1
      return fibonacci(n-1) + fibonacci(n-2)