## Sock Merchant

John works at a clothing store and he's going through a pile of socks to find the number of matching pairs. More specifically, he has a pile of loose socks where each sock is labeled with an integer, , denoting its color. He wants to sell as many socks as possible, but his customers will only buy them in matching pairs. Two socks, and , are a single matching pair if they have the same color ().

Given and the color of each sock, how many pairs of socks can John sell?

**Input Format**

The first line contains an integer, , denoting the number of socks.

The second line contains space-separated integers describing the respective values of .

**Constraints**

**Output Format**

Print the total number of *matching pairs* of socks that John can sell.

**Sample Input**

```
9
10 20 20 10 10 30 50 10 20
```

**Sample Output**

```
3
```

**Explanation**

As you can see from the figure above, we can match three pairs of socks. Thus, we print on a new line.