## Task:
Your task is to write a function which returns the sum of following series upto nth term(parameter).
    Series: 1 + 1/4 + 1/7 + 1/10 + 1/13 + 1/16 +...
## Rules:
* You need to round the answer to 2 decimal places and return it as String.
* If the given value is 0 then it should return 0.00
* You will only be given Natural Numbers as arguments.
## Examples:
    SeriesSum(1) => 1 = "1.00"
    SeriesSum(2) => 1 + 1/4 = "1.25"
    SeriesSum(5) => 1 + 1/4 + 1/7 + 1/10 + 1/13 = "1.57"
**NOTE**: In PHP the function is called `series_sum()`.
def series_sum(n):
    n = n - 1
    return f'{sum(1/i for i in range(1,2+3*n,3)):.2f}'
def series_sum(n):
    n = n - 1
    return f'{sum(1/i for i in range(1,2+3*n,3)):.2f}'