How can evaluate an expression stored as a string in F#

I want to do something sort of like this:

let x = 5
let y = 10

let expr = Console.ReadLine()

expr

Where one might type "x+y" in the console to store in expr.

How does one evaluate a statement like this in F#?

Ultimately, I want a user to be able to enter expressions, or a set of rules for a system, on a webpage and have them saved in a database to be applied at appropriate times in an F# library. I just don't know how to convert the entered string in to a function value in F#.

Thanks for any help you may provide!

Adam