This can be achieved through the filter alone.
Drag the dimension onto the filters then choose filter, in the general tab, exclude nulls (deselect all values, tick nulls then tick exclude), then on the top tab, choose "By Field:" Bottom, in the empty space choose 1 [you are choosing the bottom 1 values], the field is your dimension and aggregation is Min.
Please post a packaged workbook with some sample data, there are a variety of ways to go about this and the exact method used is dependent on your data. For example, if the dimensional value with 21600666 is always increasing alphanumerically there's one technique that could work (what Steve said), if the dimension has a different range then other technique(s) would be required.
Very good point Jonathan - apologies for not adding that to my response [was heading out to lunch at the time ]