a is a natural number. It is known that the sum of the digits of a and a+1 can be divisible by 7. What is the smallest such natural number a?
a是一个自然数，已知a与a+1的各位数字之和都能被7整除，那么这样的自然数a最小是多少．
<gadget id="calculator">70_000 - 1</gadget>
<output>69_999</output>

<result>69_999</result>