How do you set a Pandas function condition to be one record equal to itself offset by X recods?

I'm defining a simple if xxxx return y - else return NaN function. If the record, ['Product'], equals ['Product'] offset by 8 then the if condition is true. Here's an example of what the initial dataframe looks like.

In [2]: df
Out[2]: 
   Product  Sales Quarter
0  Shoes    25    12017
1  Shoes    3     22017
2  Shoes    6     32017
3  Shoes    23    42017
4  Shoes    11    12018
5  Shoes    12    22018
6  Shoes    9     32018
7  Shoes    45    42018
8  Shoes    3     12019
9  Jacket   12    12017
10 Jacket   10    22017
...
16 Jacket   23    42018
17 Jacket   44    12019

I've tried calling the record and setting it equal to itself offset by 8 using == and .shift(8). ['Product'] is a string and ['Sales'] is an integer.

def Growth (X):
    if X['Product'] == X['Product'].shift(8):
        return (1+ X['Sales'].shift(4)) / (1+ X['Sales'].shift(8) - 1)
    else:
        return 'NaN'

I expect the output to be NaN for the first 8 records, and then to have numbers at record 9 (like shown below), but I receive the error instead.

   Product  Sales Quarter Growth
0  Shoes    25    12017   NaN
1  Shoes    3     22017   NaN
2  Shoes    6     32017   NaN
3  Shoes    23    42017   NaN
4  Shoes    11    12018   NaN
5  Shoes    12    22018   NaN
6  Shoes    9     32018   NaN
7  Shoes    45    42018   NaN
8  Shoes    3     12019  -.54
9  Jacket   12    12017   NaN
10 Jacket   10    12017   NaN
...
16 Jacket   23    42018   NaN
17 Jacket   44    12019   ##

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().