MySQL - select distinct value from two column

Question

I have a table with the following structure:

IdM|IdS
-------
 1 | 2
 1 | 3
 1 | 4
 2 | 1
 2 | 3
 2 | 4
 3 | 1
 3 | 2
 3 | 3
 3 | 4

How could