input: Write a function to find the n-th Fibonacci number.
  assert fibonacciNum(8) == 21
  assert fibonacciNum(10) == 55
  assert fibonacciNum(5) == 5
output: def fibonacciNum(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