input: Write a function to calculate the nth Fibonacci number.
  assert fib(0) == 0
  assert fib(1) == 1
  assert fib(10) == 55
  assert fib(20) == 6765
output: def fib(n):
      if n <= 1:
          return n
      a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b