calculus - How does k in $int_{-infty}^infty ke^{frac{-x^2}{2}} ~dx =1$ solve to $k = frac{1}{sqrt{2pi}}$

I'm working on stats problems and am a bit rusty with calculus as I haven't worked with it in 3 years. My prof gave us an equation
$\int_{-\infty}^\infty ke^{\frac{-x^2}{2}} ~dx =1$ where XER and f(x) is a PDF to solve for k.

Could someone please explain to me how he arrived at $k = \frac{1}{\sqrt{2\pi}}$