Leave it blank.
However that wouldn't be advised.
Say you bid one dollar, and the minimum bid to appear in the USA is four dollars. All your ads could showing in Kazakhstan or wherever.
I suggest that you think hard about the countries where you are most likely to achieve success, and give each country their own campaign. It is easy to copy campaigns using AdWords Editor.
That way things like minimum bids, time of day performance and conversions can be observed and addressed at a country level.