When did Belgrade start and end being a country as a Principality of Serbia?
SELECT ?value1 ?value2 WHERE { wd:Q3711 p:P17 ?s . ?s ps:P17 wd:Q684030 . ?s pq:P582 ?value1 . ?s pq:P580 ?value2 }
belgrade: Q3711 principality of serbia: Q684030