You Do Need power plug adapter while traveling from Serbia to San Marino.

Here is Why You Need An Adapter

The outlet and plug type used in San Marino are C, F and L and in Serbia are C and F. This is how they look like.

Serbia

In Serbia, outlets of type C and F are in use.

Type C

Type F

San Marino

In San Marino, outlets of type C, F and L are in use.

Type C

Type F

Type L