How receive one-piece polygon from OSM Relation with overpass API

Question

How I can receive Polygon one-piece from OSM using Overpass API. When I used simple query like a:

[out:json]; 

rel['ISO3166-1'='PL']; 

out geom; 

I got on map polygon of country Poland, but when I tried look at output data I saw many piece of polygon and that affraid my. How can I receive polygon one-piece, because I don't understand how download and concatenate that...

I know that we can using EXPORT and then GeoJSON, but what about API..