MS SQL-Server Express licensing

Greetings

As usual i'm having trouble finding my way around in the Microsoft licensing
jungle.

I think that the easiest thing is to describe my setup and then hope someone
is able to help me.

I have 5 devices running a application on top of Windows CE.
Can i use the free version of SQL-Server or du i have to purchase some kind
of a license ? If yes, which one ?

Thanks in advance

Henrik
.