need help with my knitting

hi can somebody help me with my knitting im on with the button holes at the moment now ive got 409 sts on my needles now it says that ive got to rib to last 111 sts cast off 2sts then have to rib 18 cast off 2 sts and have to do this 5 times then at the end off the row ive got to rib 3 now i have tryed this a few times but i cant get it right some how in knitting the cardigan all together now its just that part that im stuck on it

Is it the numbers not adding up? You have to remember that in order to cast off 2 sts you have to work 3sts. So you’ll have one stitch left on the right needle after the initial cast off, then rib another 18 (19sts on the right needle after the cast off) and then cast off 2 and repeat.
2+ 5(21) +4 = 111sts.
Or is it something else that’s not working out?