How to count the number of people in a room?

Question

Algorithms

Good day!
From a technical point of view, how can you count the number of people in a room?
Suppose that it is necessary to calculate the number of people in the shopping center in real time.
Maybe you can solve the problem by installing some sensors at the entrance? But, in that case, how to take into account the people who left the building?
Any options are accepted, but priority is given to low-budget ones :)
Thank you all!

Answer 1

Plant the grandma of the "watchman" system at the entrance with two buttons for +1 and -1. And there are enough electronic people counting systems on the market - for any task, and their accuracy is acceptable, and the price depends on the type of sensors and their number.

Answer 2

https://megacount.ru/

Answer 3

https://habrahabr.ru/post/291522/