input: Write a python function to generate the nth Fibonacci number.
  assert fibonacciSequence(0) == 0
  assert fibonacciSequence(1) == 1
  assert fibonacciSequence(8) == 21
  assert fibonacciSequence(10) == 55
  assert fibonacciSequence(15) == 610
output: def fibonacciSequence(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b