So they're playing rock-paper-scissors and the winner gets to take five steps up the stairs.
When they do so, they say each syllable of the word "chocolate(cho-co-le-yee-to in Japanese)" for every step they go.
But Nagi said "chocolate is Meiji", a famous copyright meaning that Meiji's chocolates are the best, and took four more steps for their syllables.
Riamu gets the joke and mentions Glico which is a business competitor of Meiji.

Too bad it's not as fun when I explain it but you get the picture.

For all it counts, you have my gratitude. A lot of jokes are localized and it would take me too long to figure it out on my own~