So what I understand is that you start from cell 1 and put 1 in there,
in the 2nd cell it would be 2 (so you type the 1 number (1 and write "=A1+1") then copy to next cell.
once you get to number 13, erase it and in the next cell write "=A12+2"), then in next cell write the formula mention above.
This should give you what you want.
I can send you an example if you need one just write me a personal message