Question: 
Write a function to calculate the sum of perrin numbers.


Model's Code:
def cal_sum(n):
    if n < 0:
        return 0
    perrin = [3, 0, 2]
    while len(perrin) <= n:
        perrin.append(perrin[-2] + perrin[-3])
    return sum(perrin[:n+1])



Sample Correct Code:
def cal_sum(n): 
	a = 3
	b = 0
	c = 2
	if (n == 0): 
		return 3
	if (n == 1): 
		return 3
	if (n == 2): 
		return 5
	sum = 5
	while (n > 2): 
		d = a + b 
		sum = sum + d 
		a = b 
		b = c 
		c = d 
		n = n-1
	return sum


Asserts:
assert cal_sum(9) == 49
assert cal_sum(10) == 66
assert cal_sum(11) == 88


Tests:
Test 1 of 3: PASS
Test 2 of 3: PASS
Test 3 of 3: PASS

Test 448 of 974: 326/448 (72.76785714285714%): success