ZYB loves binary strings (strings that only contains `0' and `1'). And he loves

more, where the number of `0' and the number of `1' in the string are equal.
ZYB wants to choose a substring from an original string

so that it is an

with the longest length possible. He also wants to choose a subsequence of

which meets the same requirements.
A string

is a substring of a string

if

is empty, or there are two integers

and
)
such that

. A string

is a subsequence of a string
if it can be derived from
by deleting any number (including zero) of characters without changing the order of the remaining characters.
For simplicity, you only need to output the maximum possible length. Note that the empty string is both a substring and a subsequence of any string.