One electron collides elastically with a second electron initially at rest. After the collision, the radii of their trajectories are $r_1$ and $r_2.$ The trajectories are perpendicular to a uniform magnetic field of magnitude $B.$ Determine the energy of the incident electron. $($Use any variable or symbol stated above along with the following as necessary: $m_e$ for the mass of elecron and $e$ for the electron's charge.$)$

$K=$