There is a three-digit number \textquotesingle$$502$$\textquotesingle. John writes a digit before this number to make it a new four-digit number. If he writes a~\uline{~~~~~~~~~~}~, the new four-digit number would be as small as possible. 
Overseas Competition->Knowledge Point->Combinatorics->Combinatorics Involving Extreme Values->Extreme Value in Enumeration Problems
Note that the digit \textquotesingle$$0$$\textquotesingle{} could not be the highest digit of a four-digit number. Therefore, the smallest number is $$1502$$. 