input: Write a python function to return the nth Fibonacci number.
  assert fibonacci(0) == 1
  assert fibonacci(1) == 1
  assert fibonacci(5) == 8
  assert fibonacci(10) == 89
output: def fibonacci(n):
      if n == 0 or n == 1:
          return 1
      a, b = 1, 1
      for _ in range(2, n+1):
          a, b = b, a + b
      return b