Helical motion of a charged particle in a constant magnetic field

An electron (charge $-e$) enters a region with a constant magnetic field $B=$  $\hat{z}$ [T]. The intitial velocity of the electron is

The electron spirals around the $z-$axis. Viewed along the $z-$axis, the path of the electron looks like a circle. What is the radius of this circle?

$R=$ [m]