Returning Values base on a series and the highest in the series

[liverebel]

In the attached here is what im trying to do. Column 1 [Series#] contains number series that are based on year eg 17 - and then a reference number. After the reference number is another series number preceded by a period. The number that comes after the period is a series reference number with in the main series number. So 17-0012 contains 17-0012.1 17-0012.2 17-0012.3 17-0012.4 etc. I would like column C [Top Series] to return the some indication that number ending the series is the highest number in that series. So in the example of 17-0012 which contains 4 sub series I would like column 3 to put an "x" or the number "17-0012.4" ONLY for the number that is the highest in that series leave 17-0012.1 17-0012.2 17-0012.3 blank. Just some way to indicate in column C that .4 is the highest within the 17-0012 series. Thanks!