input: Write a function to calculate the nth number in the Fibonacci sequence. The function should return the Fibonacci number at position n.
  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): 
              c = a + b 
              a = b 
              b = c 
          return b 
  
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(9) == 34
  assert fibonacci(15) == 987
  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