input: Write a function to calculate the nth number in the Fibonacci sequence. If the input n is less than 0, print "Incorrect input". If n is 0, return 0. If n is 1, return 1. Otherwise, iterate from 2 to n and compute the Fibonacci number.
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(5) == 5
  assert fibonacci(10) == 55
  assert fibonacci(15) == 610
output: def fibonacci(n):
      if n < 0:
          print("Incorrect input")
      elif 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