input: Write a python function to compute the sum of the first n Fibonacci numbers.
  assert fibonacci_sum(0) == 0
  assert fibonacci_sum(1) == 1
  assert fibonacci_sum(5) == 12
  assert fibonacci_sum(10) == 143
output: def fibonacci_sum(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      
      a, b = 0, 1
      total = a + b
      for _ in range(2, n):
          a, b = b, a + b
          total += b
      
      return total