Array + loop

Hello,

$var1=(1,2,3,4,5,...m)
$var2=(1,2,3,4,5,...,n)
Do
   $var=$var1+$var2
   msgbox(0,"result",$var)
Until(ends after n desired times)

First problem:
I want $var1 & $var2 to change of value every loop.
The first time the operation will be: 1+1 and the msgbox will display 2
The second time 2+2, 4
Etc.

How can the code do that?

Let`s imagine I want the code to stop after n operations (that I determined with an inputbox), what is the condition I have to put inside until()? I imagine it is a counter but I don`t known how to count every loop...