Question: https://codeforces.net/problemset/problem/1906/M

Editorial: https://competition.binus.ac.id/icpc2023/M5xmtK1iPAdXT1t765fbLg5m66d52XuT.pdf

The second part in the editorial where $$$M \le 2 \cdot (S - M)$$$

I read it, but still feel I don't quite get it.
Can someone please help me to understand the math of the second part