Middle finger

On Saturday 26/08/17 while going through the drive through at mcdonalds I knocked on the car window (I was sitting at the back) to get the guy on the mic's attention. he looked at me and I greeted him with a thumbs up and he legit looked at me and extended his middle finger. ***?? he then proceeded to laugh. this is unacceptable!!!! his nametag said "PK".