Arrange all the two-digit numbers in ascending order, and if the numbers are equal, arrange them in ascending order of actual size (for example, the sum of "10" is 1+0=1, and the sum of "11" is 1+1=2, the sum of "20" is 2+0=2, but 11<20, so the arrangement of these 3 numbers is 101120) to a multi-digit number: 101120122130...899899. So what is the 40th digit of this multi-digit number from the front.
把所有的两位数按数字和从小到大排列，如果数字和相等的就按照实际大小从小到大排列，(比如"10"的数字和是1+0=1，"11"的数字和是1+1=2，"20"的数字和是2+0=2，但是11＜20，所以这3个数的排列为101120)这样的到一个多位数：101120122130…899899．那么这个多位数从前面数第40个数字是多少．
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