Question 16
Define an *upno* to be a positive integer of $2$ or more digits where the digits are strictly increasing moving left to right. Similarly, define a *downno* to be a positive integer of $2$ or more digits where the digits are strictly decreasing moving left to right. For instance, the number $258$ is an upno and $8620$ is a downno. Let $U$ equal the total number of upnos and let $D$ equal the total number of downnos. What is $|U - D|?$