Sattelite orbiting mars, work done to change distance.

1. The problem statement, all variables and given/known data
A 2700 kg spacecraft is in a circular orbit 1000 km above the surface of Mars. How much work must the spacecraft engines perform to move the spacecraft to a circular orbit that is 4000 km above the surface?

mars: Mass=6.4185*10^23 KG, radius=3397000 m
2. Relevant equations
KE=.5Gm1m2/r (this doesn't seem right for some reason)

3. The attempt at a solution
Work= TME2-TME1=3.92*10^9
webassign says I'm wrong. Is my equation for either KE or potential energy incorrrect? Any help is appreciated.
Are you using a negative for U ?
sure am
Sorry for replying late, I have a final for this intro mechanics tomorrow morning :)
This is the way i did it :

E1 (energy at lower orbit) = -GmM/2r1 where r1 is the radius of mars + 1000 km
E2 (energy at higher orbit)= -GmM/2r2 , r2 is the radius or mars + 1000 km

m is 2700 kg
M is mass of Mars.

E2-E1 = GmM/2 (1/r1 - 1/r2) gives 5.33 x 10^9 J
Thanks Ahmadmz, that's what I got too after I corrected my excel spreadsheet, had the wrong equation in for velocity. ( I do all my hw in excel)

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