input: Write a python function to compute the nth Fibonacci number.
  assert fibonacci(1) == 0
  assert fibonacci(2) == 1
  assert fibonacci(3) == 1
  assert fibonacci(10) == 34
  assert fibonacci(15) == 377
output: def fibonacci(n):
      if n <= 0:
          return 0
      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