Sum

So imagine you have numbers in Cells A1:A10. In A11 you want to SUM all of these, but you ONLY want to sum the cells chosen by a user, so you have cells A20 and A21 as your FROM and TO.

So, if you type 3 in cell A20, and 7 in cell A21, you want your sum function to read SUM(A3:A7). The problem is, you can't write a sum function to read:

SUM(A(A20):A(A21)) - it simply doesn't work. There has to be an easy way to do this. I'm thought about using SUMIF, but there really isn't an IF statement or operator I can use on this that I can think of. Any ideas?

There's also the non-volatile Index formula: =SUM(INDEX(A1:A10,A20):Index(A1:A10,A21))