CF95B.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.

佩佳喜欢幸运数字。众所周知，如果一个正整数的十进制表示中只包含数字 4 和 7，则该数为幸运数。例如，47、744、4 是幸运数，而 5、17、467 不是。

如果一个幸运数的十进制表示中数字 4 和数字 7 的个数相等，则称其为“超级幸运数”。例如，47、7744、474477 是超级幸运数，而 4、744、467 不是。

某天，佩佳遇到了一个正整数 $n$。请帮助他找出不小于 $n$ 的最小超级幸运数。

The only line contains a positive integer n (1 ≤ n ≤ 10100000). This number doesn't have leading zeroes.

唯一一行包含一个正整数 $n$（$1 \leq n \leq 10^{100000}$）。该数不含前导零。

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

输出大于或等于 $n$ 的最小超级幸运数。