OpenOffice Calc Compare 2 cols and print col if true

Question

I have two cols in Calc and want to compare both and print in col if it's a duplicate. Example:

A    B    C
1    2    
2    2    2
3    3    3
4    5    
5    6    
6    7    
7    7    7
8    8    8
9    9    9

What is the formula I need to print in col c to get this?

Answer 1

The formula to use in column C, to test if columns A and B are equals

=IF(A1=B1;B1;"")