I do not get the context of two hours here. AFAIK, ACM Programming contest consist of ~5 hours.
FWIW, I am also participating this year. My team was ranked #11 in prelims and we are competing for qualifying world finals at IITK, India. ( http://www.cse.iitk.ac.in/users/acm/ )
It took me two hours to solve it (though that's a reflection on me, not the problem. ;) ), which is where that number came from.