My 2018 Toyota Prius Supreme was parked too far away from the curb in front of a house where we were invited, in a residential area in Naperville IL. I entered the car to park it properly. I had my foot on the brakes and pressed the starting button for the engine to turn on. I then put the car on reverse and suddently the car accelerated uncontollably. Went on the grass, hit the neighbor's mailbox and stopped by crashing on a tree. I was in shock but luckily not injured. Other guests were present and witnessed the incident. Police was called in and issued a report. The car was declared by Progressive Insurance Co as a total loss.