Your task is to write a function which returns the sum of following series up to the nth term (parameter).
Series: 1 + 1/4 + 1/7 + 1/10 + 1/13 + 1/16 +...

• 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.

>     1 --> 1 --> "1.00"
> 

>     2 --> 1 + 1/4 --> "1.25"
> 

>     5 --> 1 + 1/4 + 1/7 + 1/10 + 1/13 --> "1.57"
> 
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def series_sum(n):
    return f'{(sum([1/(i*3+1) for i in range(0,n)])):.2f}'
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