数字 4 和 7 是幸运数字，而其他所有数字都是不幸运的。如果一个正整数在十进制表示下只包含幸运数字，则称这个正整数是幸运的。我们想知道第 $K$ 个幸运正整数。
输入格式
输入的第一行且仅有一行包含一个正整数 $K$ ($1 \le K \le 10^9$)。
输出格式
输出的第一行且仅有一行，必须包含第 $K$ 个幸运正整数。
样例
输入样例 1
1
输出样例 1
4
输入样例 2
2
输出样例 2
7
输入样例 3
3
输出样例 3
44
QOJ.ac
QOJ
Limite de temps : 1.0 s
Limite de mémoire : 32 MB
Points totaux : 70
#16978. SRETAN
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