Flight Düsseldorf Airport departs from Dusseldorf Düsseldorf Airport at 09:35 and arrives at Belgrade Belgrade Nikola Tesla Airport at 11:35.
The flighttime is about 4h 00m. The distance travelled ("as the bird flies"): 1660 Km. So the average speed is roughly 415 Km.
There is an 2 hour time difference between Dusseldorf and Belgrade