Basic Javascript question

[Euskadi]

I want to take three values in from the user and perform various basic mathematical operations on them.

I am taking input like this:
var firstnumber = prompt("blah blah...
var secondnumber = prompt("blah blah...

and then adding them together like this
var result = firstnumber+secondnumber

However this just concatenates the values as if they were strings, e.g. if you entered "7" and "3" instead of getting 10 you get 73!

I know how to add them if I hard-coded the variables, but I want the user to input them.

[Euskadi]

I ended up just doing:

firstnumber = firstnumber*1 to force it as a numerical value. Not sure if that's best practise?