Maximize 1s

You are given a binary string $S$. You are allowed to perform the following operation **at most once**: - Pick some [substring](https://en.wikipedia.org/wiki/Substring) of $S$ - Flip all the values in this substring, i.e, convert $0$ to $1$ and vice versa For example, if $S = 1\underline{00101}011$, you can pick the underlined substring and flip it to obtain $S = 1\underline{11010}011$. For the