AtCoder Beginner Contest 180 Takahashi Unevolved D Problem

D-Takahashi Unevolved Description Link
https://atcoder.jp/contests/abc180/editorial/219
With the answer code listed above,

ans+(y-1-x)//b

There is a part of , but I didn't understand the reason for doing -1 here.
Could you tell me what this means?

Thank you for your cooperation