[R] character comp

[Val]

Hi  All,
In a given data frame I  want to compare character values of two columns.
My sample data looks like as follow,

mydataframe <- read.table( text='ID  var1 var2
  R1   AA  AAA
  R2   AAA AAA
  R3    A  AAAA
  R4   AA   A
  R5   A  AAA', header = TRUE, as.is = TRUE )

For each ID, I want  create the third column "dvar" as  difference
between var1 and var2
 Row1( R1)   the "dvar" value will be -1 and the complete  desired out
put looks like as follow.

 ID    var1 var2   dvar
 R1   AA    AAA    -1
 R2  AAA  AAA      0
 R3    A    AAAA    -3
 R4   AA       A        1
 R5    A     AAA      -2

How do i do this? Any help please?
Thank you