Create Array of Letters and Numbers

Hello,

I'm trying to create a framework or array with two values in the attached spreadsheet. The idea is to create a varying number of points with a central spine. I tried to get this to work with some IF statements but I've run into a roadblock. Is there an excel function that can accomplish this grid or do I need to go with VBA.

Example 1:
Input
6 Points
Line after 4
S1, S2, S3, S4, M1
S5, S6, M2

Example 2:
Input
9 Points
Line after 3
S1, S2, S3, M1
S4, S5, S6, M2
S7, S8, S9, M3

Example 3:
Input
9 Points
Line after 3
S1, S2, S3, M1, S4, S5
S6, S7, M2, S8, S9

Any and all help is greatly appreciated!