CF146A.Lucky Ticket

Petya loves lucky numbers very much. Everybody knows that lucky numbers are positive integers whose decimal record contains only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.

Petya loves tickets very much. As we know, each ticket has a number that is a positive integer. Its length equals n (n is always even). Petya calls a ticket lucky if the ticket's number is a lucky number and the sum of digits in the first half (the sum of the first n / 2 digits) equals the sum of digits in the second half (the sum of the last n / 2 digits). Check if the given ticket is lucky.

佩蒂亚非常喜爱幸运数字。众所周知，幸运数字是指十进制表示中仅包含幸运数字 4 和 7 的正整数。例如，47、744、4 是幸运数字，而 5、17、467 则不是。

佩蒂亚也非常喜爱车票。众所周知，每张车票都有一个编号，该编号是一个正整数，其长度为 $n$（$n$ 恒为偶数）。佩蒂亚称一张车票为幸运车票，当且仅当该车票的编号是一个幸运数字，且前半部分数字之和（即前 $n/2$ 位数字之和）等于后半部分数字之和（即后 $n/2$ 位数字之和）。请判断给定的车票是否为幸运车票。

The first line contains an even integer n (2 ≤ n ≤ 50) — the length of the ticket number that needs to be checked. The second line contains an integer whose length equals exactly n — the ticket number. The number may contain leading zeros.

第一行包含一个偶数整数 $n$（$2 \leq n \leq 50$）——即需要检查的票号的长度。
第二行包含一个长度恰好为 $n$ 的整数——即票号。该数字可能包含前导零。

On the first line print "YES" if the given ticket number is lucky. Otherwise, print "NO" (without the quotes).

如果给定的车票号码是幸运号码，则在第一行输出 “YES”；否则输出 “NO”（不带引号）。

In the first sample the sum of digits in the first half does not equal the sum of digits in the second half (4 ≠ 7).

In the second sample the ticket number is not the lucky number.

在第一个样例中，前半部分数字之和不等于后半部分数字之和（4 ≠ 7）。

在第二个样例中，该票号不是幸运号码。