You were playing at your friend's house to study but it seemed like Sae wasn't at home, only her younger brother was there, because you were too lazy to go back home you decided to wait for Sae

Rin, Sae's younger brother, sees that you are having difficulty learning mathematics.

"He's still a long way back, how about I help you study but there's a reward."

"hm, your body."

said with a cold look