general relativity - The einbein in the action of a relativistic massive point particles

The action of a relativistic massive point particle moving in space-time is

$$S=-m\int d\tau \sqrt{g _{\nu \rho}\frac{dx^{\nu}}{d\tau}\frac{dx^{\rho}}{d\tau}}$$

[with Minkowski sign convention $(+,-,-,-)$]. Because of the the square root in the action, one introduces the einbein field $e$ and the action becomes

$$S=1/2\int d\tau(e^{-1}\dot{X}^{2}-em^{2}).$$

I understand this but I can't come up with the expression of the action with the einbein field myself.