collision

If two 2D circular shaped objects (with different velocities and masses)
collides total elastically I find that there are four unknown variables,
the two vellocities and their direction in x- and y-direction.
With conservation of momentum (mass times velocity) I get two equations, and
conservation of energy gives a third equation.
Then there is one degree of freedom left. How can I resolve the situation
and find the velocities after the collision?
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