There is a fourteen-digit number, the highest digit is 5, the ones digit is 7, and the billion digit is a. The sum of every three adjacent digits is 18. Find this number.
有一个十四位数，最高位是5，个位是7，亿位是a，每三个相邻数字和为18，求此数．
<result>56_756_756_756_767</result>