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Slava and tanks 877C

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C. Slava and tanks time limit per test 2 seconds memory limit per test 256 megabytes input standard input output standard output

Slava plays his favorite game "Peace Lightning". Now he is flying a bomber on a very specific map.

Formally, map is a checkered field of size 1?×?n, the cells of which are numbered from 1 to n

, in each cell there can be one or several tanks. Slava doesn‘t know the number of tanks and their positions, because he flies very high, but he can drop a bomb in any cell. All tanks in this cell will be damaged.

If a tank takes damage for the first time, it instantly moves to one of the neighboring cells (a tank in the cell n

can only move to the cell n?-?1, a tank in the cell 1 can only move to the cell 2). If a tank takes damage for the second time, it‘s counted as destroyed and never moves again. The tanks move only when they are damaged for the first time, they do not move by themselves.

Help Slava to destroy all tanks using as few bombs as possible.

Input

The first line contains a single integer n (2?≤?n?≤?100?000) — the size of the map.

Output

In the first line print m — the minimum number of bombs Slava needs to destroy all tanks.

In the second line print m integers k1,?k2,?...,?km. The number ki means that the i-th bomb should be dropped at the cell ki.

If there are multiple answers, you can print any of them.

Examples input Copy
2
output Copy
3
2 1 2
input Copy
3
output Copy
4
2 1 3 2
題意:炸坦克,如果炸第一個坦克,它只能跑到第二個坦克,如果炸第n個坦克,它只能跑到第n-1個坦克,如果炸中間坦克,它可以跑到它相鄰的坦克上
分析:先炸偶數位上的坦克(因為偶數總是比奇數少),然後偶數位上的坦克都跑到奇數位上了,再炸奇數位上的坦克,原來偶數位上的坦克就沒了,奇數位上的所有坦克跑到偶數位上去了,然後再炸一次偶數位上的坦克,這下所有的坦克都炸沒了
 1 #include<cstdio>
 2 int main()
 3 {
 4     int n;
 5     while(~scanf("%d",&n))
 6     {
 7         printf("%d\n",3*n/2);//總次數
 8         for(int i=2;i<=n;i+=2)
 9         {
10             printf("%d ",i);
11         }
12         for(int i=1;i<=n;i+=2)
13         {
14             printf("%d ",i);
15         }
16         for(int i=2;i<=n;i+=2)
17         {
18             printf("%d ",i);
19         }
20         printf("\n");
21     }
22     return 0;
23 }

Slava and tanks 877C