That's if they don't strike first. And keep in mind, you'll probably have the US, S.K, Australia (possibly), England (who've committed aircraft to help out with ID before, IIRC), engaged as well. And that's if a UN force doesn't decide to intervene.
There's also the issue of where China stands on this.
id say China will sit on the fence, N Koria has pissed them of on a number of occasions but they also rattle the west's cage quite frequently which suits China. thing is the south korea's standing army is about half of north korea's and the stationed foreign troops dont make up the numbers so if N Korea strikes first then you are looking at maybe days before the north overruns the south, to little time for a multi national force to respond
State-----------Active-----Reserve----Paramilitary---Total
China-----------3,440,000--1,200,000--4,100,000------8,740,000
United States---1,473,900--1,458,500--453,000--------3,385,400
India-----------1,414,000--1,800,000--1,300,300------4,514,300
North Korea-----1,106,000--4,700,000--189,000--------5,995,000
Russia----------1,037,000--2,400,000--359,100--------3,796,100
South Korea0----540,000----3,000,000--39,500---------3,579,500
Germany---------284,500----358,650----40,000---------683,150
France----------259,050----419,000----101,400--------779,450
United Kingdom--240,200----195,300----0--------------435,500
Japan-----------239,900----44,400-----12,250---------296,550
Italy-----------230,350----65,200-----238,800--------534,350
Australia-------53,572-----20,300-----0--------------73,872