Math with user variables

Hi everybody:
I have a problem about use variables. I 've created this variable:

var=`wc -l file.txt | cut -c 1-2`;
n_var="$var"-1 ;
echo $n_var;

In my case var is 8.
When echo shows $n_var does not appear as I want. The question is how can I subtract, this is operate, to my variable.
Thanks in advance and cheers.

This is not the right umbrella to post this question !!!

something like this,

var=`wc -l < filename`
echo $var
var=$(($var - 1))
echo $var