I need a formula...[RESOLVED]

[Br1an_g]

ok,

Pretty sure this is the right forum, as its more maths than Excel.
So here it is..i cant get my head around this even though i can see the pattern.

so:
If i select 2, i want to return 12 and 20
If i select 3, i want to return 19 and 27
if i select 4, i want to return 26 and 34
if i select 5, i want to return 33 and 41

the lower number difference is increasing by 1 for each increase in selected value
the higher number difference is decreasing by 1 for each increase in selected value.

e.g.

                 -4 = 12
2*8 = 16 - 
                 +4 = 20

                 -5 = 19
3*8 = 24 - 
                 +3 = 27

i just cant get my head around putting this into a useable code.
the below is for week2. i need a way to calculate the end numbers from what i already have.

StartRow = (SelectedWeek * 8) - 4
EndRow = (SelectedWeek * 8) + 4

TIA for any help