Score : $300$ points

问题描述

你有 $N$ 只袜子。第 $i$ 只袜子的颜色是 $A_i$。

你想尽可能多地执行以下操作。最多能执行多少次？

• 选择两只颜色相同且尚未配对的袜子，将它们配对。

以上为通义千问 qwen-max 翻译，仅供参考。

Problem Statement

You have $N$ socks. The color of the $i$-th sock is $A_i$.

You want to perform the following operation as many times as possible. How many times can it be performed at most?

• Choose two same-colored socks that are not paired yet, and pair them.

Constraints

• $1\leq N \leq 5\times 10^5$
• $1\leq A_i \leq 10^9$
• All values in the input are integers.

Input

The input is given from Standard Input in the following format:

$N$

$A_1$ $A_2$ $\dots$ $A_N$

Output

Print an integer representing the answer.

Sample Input 1

6
4 1 7 4 1 4

Sample Output 1

2

You can do the operation twice as follows.

• Choose two socks with the color $1$ and pair them.
• Choose two socks with the color $4$ and pair them.

Then, you will be left with one sock with the color $4$ and another with the color $7$, so you can no longer do the operation. There is no way to do the operation three or more times, so you should print $2$.

Sample Input 2

1
158260522

Sample Output 2

0

Sample Input 3

10
295 2 29 295 29 2 29 295 2 29

Sample Output 3

4

update @ 2024/3/10 12:15:54