input: Write a python function to compute the nth Fibonacci number.
  assert computeFibonacci(0) == "Incorrect input"
  assert computeFibonacci(1) == 0
  assert computeFibonacci(2) == 1
  assert computeFibonacci(5) == 3
  assert computeFibonacci(10) == 34
output: def computeFibonacci(n):
      if n <= 0:
          return "Incorrect input"
      elif n == 1:
          return 0
      elif n == 2:
          return 1
      else:
          a, b = 0, 1
          for _ in range(n - 2):
              a, b = b, a + b
          return b