input: Write a function called `fibonacci_seq` that takes an integer `n` as an argument and returns the `n`-th number in the Fibonacci sequence. The Fibonacci sequence is a series of numbers where a number is the sum of the two preceding ones, usually starting with 0 and 1. 
  
  assert fibonacci_seq(0)==0
  assert fibonacci_seq(1)==1
  assert fibonacci_seq(5)==5
  assert fibonacci_seq(10)==55
  assert fibonacci_seq(20)==6765
output: def fibonacci_seq(n):
      if 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