input: Write a function to calculate the nth number in the Fibonacci sequence. If the input is less than 0, print "Incorrect input". If the input is 0, return 0. If the input is 1, return 1. Otherwise, use a loop to compute the Fibonacci number.
  def fibonacci(n): 
      a = 0
      b = 1
      if n < 0: 
          print("Incorrect input") 
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      else: 
          for i in range(2,n+1): 
              c = a + b 
              a = b 
              b = c 
          return b 
  
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(9)==34
  assert fibonacci(15)==610
  assert fibonacci(20)==6765
output: def fibonacci(n): 
      a = 0
      b = 1
      if n < 0: 
          print("Incorrect input") 
      elif n == 0: 
          return a 
      elif n == 1: 
          return b 
      else: 
          for i in range(2,n+1): 
              c = a + b 
              a = b 
              b = c 
          return b