create a string from first 2 elements in dataframe and add to new column in R

I think this is fairly simple, say I have a data frame

> vadata 
    va1 va2
1   a   c
2   b   d

I want to create a new column va3 = "a-c" and "b-d" for all rows in vadata and append to vadata?

vadata is of type dataframe.

Is there an easy to way do this in r?