Huseyn's game

Huseyn has a string consisting of characters $0$ and $1$. To entertain himself, he came up with the following game. Huseyn can perform one of two operations on the string:

• append $0$ to the left end, and $1$ to the right end;

• append $0$ to the right end, and $1$ to the left end;

For example, from the string $010$, Huseyn can obtain $00101$ or $10100$.

You are given a string obtained after all of Huseyn's operations (it is possible that he did not perform any operation). Determine the smallest possible length the string could have initially had.