如何在SUM公式中将文本值视为MS excel中的特定数字

如果问题重复,我深表歉意。我尝试搜索,但找不到。

我在Excel中有专栏,例如:

a   | b   | c    | total
10  | 5   | 5    | 20
5   | -   | 2    | 7
4   | -   | -    | 4

I want to write a formula for total as sum =SUM(a,b,c). But as - is not a number (excel treating it as subtract sign probably) it doesn't provide desired results. I need to somehow consider NA as 0 in SUM.

Note: I can not use 0 in place of NA. It is requirement to fill absent data as -.