boom gate counting sensor

I am looking for a sensor that can be installed in a car park boom gate. This sensor will then inform us how many times the entrance boom gate goes up and down as well as the exit boom gate. with this data, i need to create a software which for example there's 200 parking lots in a building every time a car enters the parking lot boom gate goes up and down 1 lot is minus (-) and every time a car exits boom goes up and down 1 lot is added (+). I am looking for low cost solution to provide how many parking lot is available at any given time, real time, in a parking lot. I need the hardware and software associated with it.