input: Write a python function to compute the nth Fibonacci number.
  assert fib(0) == 1
  assert fib(1) == 1
  assert fib(2) == 2
  assert fib(3) == 3
  assert fib(4) == 5
  assert fib(5) == 8
  assert fib(8) == 34
output: def fib(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