Bob and the Examination

Bob is appointed as an invigilator for an examination. In the examination hall there are $N$ students with roll number $1$ to $N$ and $N$ benches. The seating arrangement is as follows: first all odd roll number students are seated and then all even roll number students. Bob wants to know the roll number of the student sitting on $K^{th}$ bench. Since Bob is weak at maths he needs your help. ***