Calculate the work to pump water out of tank

An aquarium 2 m long, 1 m wide, and 1 m deep is full of water. Find the work needed

to pump half the water out of the aquarium. (The density of water, p, is 1000 kg/m3,

and acceleration due to gravity, g, is 9.8 m/s2.)

So in order to truly understand how to do these kind of problems I decided to find different ways to solve this problem. Since you can integrate with respect to dx or dy, for dx i was wondering if you could integrate with respect to the 2m side like so

*edit..ugh its not letting me post the pictures so i'll just give the link...http://img19.imageshack.us/img19/2024/boxid.gif

from x=0 to x=1. But when i do, i get the wrong answer, 4900 J. The answer should be 2450 J =/