Chef and his Job

Chef has landed a new job of security in the infamous The Louvreb art gallery the gallery has n painitngs each of them numbered 1 to n serial wise from left to right in a wall but there are two screens between painitngs numbered a ,a+1 and b,b+1. These screens show footage of a camera overlooking the wall itself hence the screen shows the walls and the screen itself which inturn shows the wall wit