Creating an IF statement in Python that looks at previous IF statement output

I am having difficulty creating an IF statement that does the following:

• If C1 = Buy, then Buy
• If C2 = Sell, then Sell
• If C1 & C2 = nan, then current cell = previous cell

Please see example below. I am hoping to create a column like 'C3'.

Sample Dataset:

index  C1    C2
0      Buy   nan
1      nan   nan
2      nan   Sell
3      nan   nan
4      Buy   nan
5      nan   Sell
6      nan   Sell
7      nan   nan
8      nan   nan
9      Buy   nan
10     nan   Sell

Output:

index  C1    C2    C3
0      Buy   nan   Buy
1      nan   nan   Buy
2      nan   Sell  Sell
3      nan   nan   Sell
4      Buy   nan   Buy
5      nan   Sell  Sell
6      nan   Sell  Sell
7      nan   nan   Sell
8      nan   nan   Sell
9      Buy   nan   Buy
10     nan   Sell  Sell