Почему 6?

Question

def func (x):
res=0
for i in range (x):
res+=i
return res
print(func(4))

Answer 1

range(4) is 0, 1, 2, 3

so res =
0 + 0 = 0
0 + 1 = 1
1 + 2 = 3
3 + 3 = 6

so returned 6