You are given array of integers, your task will be to count all pairs in that array and return their count.

**Notes:**
   
* Array can be empty or contain only one value; in this case return `0` 
* If there are more pairs of a certain number, count each pair only once. E.g.: for `[0, 0, 0, 0]` the return value is `2` (= 2 pairs of `0`s)
* Random tests: maximum array length is 1000, range of values in array is between 0 and 1000


## Examples

```
[1, 2, 5, 6, 5, 2]  -->  2
```
...because there are 2 pairs: `2` and `5`


```
[1, 2, 2, 20, 6, 20, 2, 6, 2]  -->  4
```

...because there are 4 pairs: `2`, `20`, `6` and `2` (again)