Write the natural numbers 1, 2, 3, ... in order to form a number: 12345678910111213 ..., if when writing a certain natural number, the formed number is exactly divisible by 72 for the first time, then what is this natural number?
将自然数1、2、3，…依次写下去组成一个数：12345678910111213…，如果写到某个自然数时，所组成的数恰好第一次能被72整除，那么这个自然数是多少？
<result>36</result>