Problem of extrusion with mirror This is my mesh (the cube) and the goal is to extrude the upper part



The idea is to get THIS FINAL RESULTS BELOW:



In order to achieve this goal, I used ,as always ALT + S, but the result was this one:



My question is: Alt + s works perfectly with all the mesh i used before: why in this case, with a mirror, i get that distorsion? Is there a way to fix it? 

Thank you!
You need to switch "even thickness ON", after command starts...

eg: after pressing ALTS...



then pressing S or ALT, thus getting:



which it seems to be what you wish to obtain...