Adding the same action to every second line

Which please?

if number you want to add is on odd row, then you can use
=A1 + MOD(ROW(),2)

if you want to add for those rows which are on even row then you can use
=A1 + MOD(ROW()+1,2)

Thanks! Its absolutely clear now!

Sorry I missed the "5" part

You can do this

=A1 + (5* (MOD(ROW(),2)))

Row() is a function that gives the row number on which the function is called

mod is a function that returns a remainder. It has two parameter. First the number that you want to diviide and 2nd the number by which you want to divide

So mod(4,2) is saying divide 4 by 2 and return me the remainder which is 0

mod (1,2) is saying that divide 1 by 2 and return me the remainder which is 1

so dividing any number by 2 tells me if a row is odd (remainder would be 1) or even (remainder would be 0)

Any thing multiplied by 1 remains same, and any thing mulitiplied by 0 becomes 0

so 5 * 1 would be 5
and 5 * 0 would be 0
that answer i am adding to A1 to get the answer

hope it clarifies for you