UDF - Integer NOT in range

Hi

I'm trying to create a function that chooses an "available" integer.

Basically I want a function that returns an integer less than 10, that doesn't appear in the range A1:A10.

If A1:A10 contained the numbers 1-5 in ascending order, I want this function to return any value within 6-10. If 6 is entered in the range A1:A10 the function returns 7-10 and so forth.

It should be really simple, but I can't get my head around it. Professor Google isn't very helpful on the subject.

Any help would be appreciated.