Particle physics - invariant mass question

[tigger88]

Homework Statement

At HERA 30 GeV electrons collided head on with 820 GeV protons. Calculate the invariant mass of ep collisions.
(masses: e=0.0005GeV, p=0.938GeV)

Homework Equations

M^2 = (E1 + E2)^2 - (p1 + p2)^2 ?

The Attempt at a Solution

I know the numerical answer to this question is 314GeV, but so far I haven't been able to get it. I've tried plugging in the known values to the above equation, using E^2 = p^2 + m^2 to find momentum, but obviously didn't get the right answer.

What should I be doing?

Thanks!

 

[Fightfish]

Hint: Consider direction of their momentum.

 

[tigger88]

Wow. That was really stupid of me! Thanks for the hint! Got it now :)