What's wrong with the batch file?

Question

cmd/bat

All pictures in the folder must be added to the beginning of a random number. What's wrong?

set MIN=999
set MAX=9999


for /f %%a in (*.jpg) do (
  @SET /a n=%MIN%+(%MAX%-%MIN%+1)*%random%
  ren "%%~dpa*.jpg" "%%%n%a.jpg"
)

pause

Answer 1

A lot of things are wrong :-(
Create 2 files written below and run the first
file _runme.cmd
file process.cmd

for %%a in (*.jpg) do (
  SET /a n=1000+!RANDOM!*8999/32768
  ren "%%a" "!n!%%a"
)
pause