input: def fibonacci_seq(n: int) -> int:
      """Given an integer n, return the nth Fibonacci number. The Fibonacci sequence is defined by the recurrence relation: F(n) = F(n-1) + F(n-2) where F(0) = 0 and F(1) = 1.
      >>> fibonacci_seq(0)
      0
      >>> fibonacci_seq(1)
      1
      >>> fibonacci_seq(5)
      5
      >>> fibonacci_seq(10)
      55
      """
output:     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