input: Write a python function to generate the nth Fibonacci number.
  assert fib(0)==0
  assert fib(1)==1
  assert fib(5)==5
  assert fib(10)==55
  assert fib(15)==610
output: def fib(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