DISTINCT ON () with UNION

How can you use a distinct on () including the whole union.

eg

select distinct on (valutaid) valutaid, short from valuta UNION select
landid, land from land order by valutaid;

table: valuta
valutaid valuta
1 USD
2 SEK

table: land
landid land
1 Sweden
2 USA
3 Norway

The above would give
1 USD
1 Sweden
2 SEK
2 USA
3 Norway

What I would like to get is
1 USD
2 SEK
3 Norway

I did specify distinct on (valutaid) anyways =)

Any suggestions?

Best regards
Per-Olof Pettersson