Exiting from a magnetic field

The above shows the path (green arrows) of a charged particle in an $xy$ -plane, where a uniform magnetic field exists penetrating through the $xy$ -plane for the left half. In the beginning when the charged particle is inside the magnetic field, the path is horizontal in the $+x$ direction. After getting out the magnetic field, at the height of $H$ , the path becomes a projectile motion, ending on the ground at last. If the magnetic field intensity is $B$ , the mass of the charged particle is $m,$ and the quantity of the electric charge of the particle is $q,$ then what is the distance $L$ above? (Gravitational acceleration is $g$ , and the amount of air resistance is negligible.)