Just the do mathematics & it's simple (Code in C# )-

            public static int pageCount(int n, int p)
                    {
                            int shortestRoute = 0;
                            if(n/2 < p){
                                    //Start from back side
                                    if(n%2 != 0){
                                            shortestRoute = (int)(n-p)/2;    
                                    }else{
                                            shortestRoute = (int)((n-p)+1)/2;
                                    }   
                            }
                            else{
                                    //Start from biggining
                                    shortestRoute = (int)p/2;
                            }

                            return shortestRoute;
                    }