Swipe to rotate an object and when it reach 30° snap it to 90°

The problem I wanted to solve :
When I swipe the screen in the right direction,
the cube starts to rotate on the Y axis,
and when it reach 30°, the cube y rotation snaps smoothly to 90°.
The same for the left direction with negative values.
And by smoothly i mean a fast animation to 90° and not abrupt change to it. But this is a second issue, not the most important.

I tried and combined a lot of codes in vain, read about quaternion and eulers yet couldn't figure a proper solution.
I hope you guys could help me with your knwoledge as am new to unity. Thank you very much.