CF534A.Exam

An exam for n students will take place in a long and narrow room, so the students will sit in a line in some order. The teacher suspects that students with adjacent numbers (i and i + 1) always studied side by side and became friends and if they take an exam sitting next to each other, they will help each other for sure.

Your task is to choose the maximum number of students and make such an arrangement of students in the room that no two students with adjacent numbers sit side by side.

一场考试将在一个狭长的教室中举行，共有 $n$ 名学生参加，因此学生们将按某种顺序排成一列就座。老师怀疑编号相邻的学生（即编号为 $i$ 和 $i+1$ 的学生）总是一起学习、关系密切；若他们在考试时恰好坐在相邻座位上，则一定会互相帮助。

你的任务是：选出尽可能多的学生，并安排他们在教室中的座位，使得任意两名编号相邻的学生都不坐在相邻的座位上。

A single line contains integer n (1 ≤ n ≤ 5000) — the number of students at an exam.

一行包含一个整数 $n$（$1 \leq n \leq 5000$）——考试的学生人数。

In the first line print integer k — the maximum number of students who can be seated so that no two students with adjacent numbers sit next to each other.

In the second line print k distinct integers _a_1, _a_2, ..., a__k (1 ≤ a__i ≤ n), where a__i is the number of the student on the i-th position. The students on adjacent positions mustn't have adjacent numbers. Formally, the following should be true: |a__i - a__i + 1| ≠ 1 for all i from 1 to k - 1.

If there are several possible answers, output any of them.

第一行输出整数 $k$ —— 满足条件的最大学生人数，即任意两个编号相邻的学生不能坐在相邻的位置上。

第二行输出 $k$ 个互不相同的整数 $a_1,\,a_2,\,...,\,a_k$（其中 $1\le a_i\le n$），表示第 $i$ 个位置上所坐学生的编号。相邻位置上的学生编号不能相邻。形式化地，对所有 $i$ 从 $1$ 到 $k-1$，需满足 $|a_i - a_{i+1}| \ne 1$。

若存在多种可能的答案，输出任意一种即可。