start in recovery mode (power+volume up+home buttons)

restore to factory mode (format data and cache partitions)

format system partition

mount system partion

sideload cm-11-20141115-SNAPSHOT-M12-i9100.zip (latest cm11 for s2 I could find) && gapps-kk-20140105-signed.zip (with gapps-kk-20140606-signed.zip there are some problems) (prefer adb method)

reboot