Someone has to suggest brute force with array formulas.
M2:
Fill M2 down as far as needed. Enter array formulas by holding down [Ctrl] and [Shift] keys before pressing [Enter].
The array in the formula above could be generated by
so that if you wanted sums of 3 columns it'd change to
or if you wanted the sums of 5 columns from F2:Y2, it'd change to
The number of adjacent columns to sum (n) become the divisor inside the INT call, and the ending array are sequential integers from 0 to the total number of columns divided by n less 1.
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