I am not able to understand how the final result is 6

Question

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

print(func(4))

Answer 1

range(4) = 0 , 1 , 2 , 3
for i in range(x): res += i
So it mean res is summary of range(4) = sum(0, 1, 2, 3) = 6