A8247. Lucky Numbers

Petya loves lucky numbers. Everybody knows that positive integers are lucky if their decimal representation doesn't contain digits other than $4$ and $7$ . For example, numbers $47$ , $744$ , $4$ are lucky and $5$ , $17$ , $467$ are not.

Lucky number is super lucky if it's decimal representation contains equal amount of digits $4$ and $7$ . For example, numbers $47$ , $7744$ , $474477$ are super lucky and $4$ , $744$ , $467$ are not.

One day Petya came across a positive integer $n$ . Help him to find the least super lucky number which is not less than $n$ .

The only line contains a positive integer $n$ ( $1<=n<=10^{100000}$ ). This number doesn't have leading zeroes.

Output the least super lucky number that is more than or equal to $n$ .