Assigning a variable based on a formula in VBA Excel

I'm trying to do a simple a macro that runs a formula and displays it in a msgbox. Right now I have:

Sub basic_messagebox()
    Dim CAT1 As Integer
    MsgBox "The total number of possible Cat I: " & CAT1
End Sub

What I don't have is a value set for CAT1. Basically I need it to run the formula =COUNTIF(F2:F1000,"x") and set it as the variable CAT1. I've been googling all over but most of what I'm finding is just people trying to use macros to set formulas in cells, which is the opposite of what I need.

What do I need to add to my little msgbox formula to set the above formula as the variable CAT1?