Re-entering the EU after a visit to the UK

Hi

I'm currently in Moscow, and I'm going to be entering the European Union soon. I would just like to know if, that once in the EU, am I allowed to visit the UK, then come back into Europe? My plan is to travel from Paris to London, and after my visit to the UK, fly into Prague.

Also, can the same be done for non-EU countries, such as Serbia? I was thinking of going from Croatia to Serbia and then back to Croatia.

I know that I'm allowed to be in the EU for 90 days. I hope that crossing between EU and non-EU countries won't distrupt that in any way.

Thanks if you can help
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