I can not solve the following problem,

Let say we have a monthly discharge data of 20 years written in Col A1 to
A240.
Now I want to have the yearly data as a output in (C1 : C20) for 20 years,
where C1 will be sum of col A1:A12, C2 will be sum of colA13:A24 and so on.

I write the formula in C1 = sum(A1:A12). After when i drag the cell downward
the col C2 to C20 is not giving the value correctly, C2 only calculates the
sum of A2 to A13 ie A2:A13 (adding 1 to A1 and 1 to A12) and not the correct
sum of A13:A24. How to figure it out??

Thanks!!!!!!