Suppose
* n in uneven,
* the first (n-1)/2 seats and the last (n-1)/2 seat has the same number of people already sitting and
* the person sitting in seat (n+1)/2 arrives.
The description does not say if he will choose left or right.
I got AC by assuming he will not choose the same side as Vasya.

1 ≤ k ≤ n ≤ 50; n is even