title: Add/subtract variables in a really dumb shell
body: <p>I am writing a shell script which works on my local <code>/bin/sh</code> fine (dash on Ubuntu 13.04), but I unltimately need to run it on a dumb box where I'm getting an error because of an operation on variables:</p>
  
  <pre><code>$((n2 - n1 + 1))
  </code></pre>
  
  <p>doesn't work, I get an error like:</p>
  
  <pre><code>syntax error: you disabled math support for $((arith)) syntax
  </code></pre>
  
  <p>I don't know a lot about the <code>sh</code> on there but I think this thing is busybox.  How can I do maths on this dumb shell?</p>
  
  <hr>
  
  <p><em><strong>edit</em></strong> with list of applets</p>
  
  <pre><code>~ # busybox --list
  [
  arp
  ash
  cat
  chgrp
  chmod
  chown
  chroot
  chvt
  clear
  cmp
  cp
  cut
  date
  dd
  deallocvt
  df
  dmesg
  du
  echo
  env
  false
  find
  freeramdisk
  ftpget
  ftpput
  grep
  gunzip
  gzip
  hexdump
  hwclock
  ifconfig
  ln
  losetup
  ls
  md5sum
  mkdir
  mkfifo
  mknod
  mkswap
  more
  mount
  mv
  nslookup
  ping
  ping6
  ps
  pwd
  renice
  reset
  rm
  rmdir
  route
  seq
  sh
  sha1sum
  sha256sum
  sleep
  sort
  swapoff
  swapon
  switch_root
  sync
  tar
  taskset
  tee
  telnet
  test
  tftp
  time
  top
  touch
  true
  umount
  uname
  uniq
  uptime
  usleep
  vconfig
  vi
  wget
  whoami
  yes
  </code></pre>