input: Write a python function to calculate the sum of the first n Fibonacci numbers.
  assert fib_sum(5) == 12
  assert fib_sum(6) == 20
  assert fib_sum(7) == 33
output: def fib_sum(n):
      a, b = 0, 1
      total = 0
      for _ in range(n):
          total += b
          a, b = b, a + b
      return total