The drone show

Priya's company is flying a drone light show for the city's New Year festival. Before take-off, the drones line up on the ground in neat rows.

In rows of 10, the last row has only 9 drones: one short. In rows of 9, the last row is one short too. The same happens with rows of 8, 7, 6, 5, 4, 3 and even 2. Every time, the last row is exactly one drone short.

Rows of 11 do not come out even either. In the end, Priya lines them all up in single file.