Subtract two variables in Bash

I have the script below to subtract the counts of files between two directories but the COUNT= expression does not work. What is the correct syntax?

#!/usr/bin/env bash

FIRSTV=`ls -1 | wc -l`
cd ..
SECONDV=`ls -1 | wc -l`
COUNT=expr $FIRSTV-$SECONDV  ## -> gives 'command not found' error
echo $COUNT