Duizi and Shunzi

Nike likes playing cards and makes a problem of it.

Now give you n integers, $a_i (1 \le i \le n) $

We define two identical numbers (eg: $ 2,2 $) a Duizi,
and three consecutive positive integers (eg: $ 2,3,4 $) a Shunzi.

Now you want to use these integers to form Shunzi and Duizi as many as possible.

Let s be the total number of the Shunzi and the Duizi you formed.

Try to calculate $max(s)$.

Each number can be used only once.

The input contains several test cases.

For each test case, the first line contains one integer n($ 1 \le n \le 10^6$).
Then the next line contains n space-separated integers $a_i$ ($1 \le a_i \le n$)

For each test case, output the answer in a line.

7
1 2 3 4 5 6 7
9
1 1 1 2 2 2 3 3 3
6
2 2 3 3 3 3 
6
1 2 3 3 4 5

2
4
3
2

Case 1（1，2，3）（4，5，6）

Case 2（1，2，3）（1，1）（2，2）（3，3）

Case 3（2，2）（3，3）（3，3）

Case 4（1，2，3）（3，4，5）

2017ACM/ICPC广西邀请赛-重现赛（感谢广西大学）