[kucheravka]

I had a hard time but finally I got it after reading this part

Suppose you're on a game show and you're given the choice of three doors [and will win what is behind the chosen door]. Behind one door is a car; behind the others, goats [unwanted booby prizes]. The car and the goats were placed randomly behind the doors before the show. The rules of the game show are as follows: After you have chosen a door, the door remains closed for the time being. The game show host, Monty Hall, who knows what is behind the doors, now has to open one of the two remaining doors, and the door he opens must have a goat behind it. If both remaining doors have goats behind them, he chooses one [uniformly] at random. After Monty Hall opens a door with a goat, he will ask you to decide whether you want to stay with your first choice or to switch to the last remaining door. Imagine that you chose Door 1 and the host opens Door 3, which has a goat. He then asks you "Do you want to switch to Door Number 2?" Is it to your advantage to change your choice? What happens is if your initial selection is a door with goat (say door 1) then the host must show the goat in the remaining doors (2 and 3) and the other door has to have the car, so if you switch you will get the car.

This won't work if you initial selection was the car, because the two remaining doors are goats and the host can open either door.

But the initial chance of geting a goat from the first selection is 2/3, so the probability of getting a car is higher if you always switch.