How do you add difficulty at suprnova?

z-enemy -a x16r -o stratum+tcp://rvn.suprnova.cc:6667 -u your_Login.your_worker_name -p x -i 20

Is this correct: z-enemy -a x16r -o stratum+tcp://rvn.suprnova.cc:6667 -u your_Login.your_worker_name -p x -i 20 -d 16

?