Sequential letters in a formula

Hi and welcome to the forum!

Unfortunately you've given very little information.

You say "and so on", but that doesn't tell us what results should be returned once we reach the limit of either the first row or first column in your matrix (whichever happens first). Do we then take sums from other rows/columns, e.g. pairings of entries from column B with entries from row 2?

Anyway, the best I can do is to give you this solution and trust that you are able to tailor it to your needs: based on a matrix in A1:E5, this formula in your first cell of choice:

=SUM(INDEX($A$1:$E$5,ROWS($1:1),1),INDEX($A$1:$E$5,1,1+ROWS($1:1)))

which will give the equivalent result to:

=SUM(A1,B1)

and, when copied down to further rows, the equivalent to:

=SUM(A2,C1)
=SUM(A3,D1)
=SUM(A4,E1)

...etc.

Regards

Or the same result, but using OFFSET, instead...

=OFFSET($A$1,ROWS($1:1)-1,,)+OFFSET($A$1,,ROWS($1:1),)