N[e^-1-sum[2^n e^(-2n-1)/n LaguerreL(n-1,1,n+1),{n,1,12}]]

Uh oh! Wolfram|Alpha doesn't run without JavaScript.

Please enable JavaScript. If you don't know how, you can find instructionshere.Once you've done that, refresh this page to start using Wolfram|Alpha.