This is the problem url: https://codeforces.net/problemset/problem/1906/M

I can't understand the editorial of this problem. Specifically I don't understand the second scenario, how can we conclude M <= 2 * (S — M) will hold after one move. According to me, if the value of M is reduced by 0 or 1 and S is reduced by 3, this condition should not hold.