Divide last row in each row of my columns

This is my dataframe:

    Deta <- c("2010-01-29" , "2015-01-29", "2017-01-30")
    v <- c(5.1, 3, 4)
    a <- c(4.3, 2, 6)
    z <- data.frame(Deta,v,a)

I want for columns v and a in each row this formule (last row / row -1). Example ( 4 / 5.1 -1) Could help me ?

I use this mutate_at(-1, c(z[last(z)])/z-1) but i have an error

This is the output:

    Date           a         v
    2010-01-29  -0.21569  0.3953
    2015-01-29  0.33333   2.0000
    2017-01-30  0.00000   0.00000