input: Write a function to generate the nth Fibonacci number.
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(10) == 55
  assert fibonacci(15) == 610
output: def fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for _ in range(2, n + 1):
              a, b = b, a + b
          return b