rounding 1 to have two floating digits

Kind of non-trivial but I tried to use round(1,2) to show 1.00 but it shows 1.0, what can I use to show 1.00 in Python?

def series_sum(n):
    # Happy Coding ^_^
    sum = 0
    for i in range(n):
        sum += 1/(1+(3*i))
    return round(sum, 2)

This is a return value for a coding challenge in codewars not a print. so it is supposed to be return and you just write the method.