Fake Certificate

You are given a binary string S of length N, representing the attendance sheet of Alice.

For the days when Alice went to school, Si=1, otherwise, Si=0

Alice wanted to increase her attendance. She can perform the following operation at most once :

• Choose any substring of S where Alice was absent everyday. She can then give a medical certificate for this period and will be marked present for the whole period .

You need to find the maximum number of days Alice will be marked present after performing the operation at most once .