setting dynamic range for adding values

Hi
I was wondering if someone can help me. i am trying to add all values in column A until cell value is Last. i.e.
A B C D
1 54 Total 173
2 45
3 62
4 12
5 last
6 452

so the formula i have which does not work is this: =SUM("A1:a"&MATCH("last",A:A,0)-1)
i was wondering if anyone can suggest an improvement to this.

Thank you in advance.