PROGRAMMING:Reverse order pair
Given an integer sequence of length N, please calculate the number of reverse order pairs in the sequence.
The definition of reverse order pair is as follows: for the i-th and j-th elements of a sequence, if I < J and a [i] > a [J], it is a reverse order pair; Otherwise it's not.
###Input format:
The first line contains the integer n, which represents the length of the sequence; The second line contains n integers, representing the entire sequence（ 1≤n≤100000）
###Output format:
Output an integer that represents the number of reverse order pairs.
###Input example:
```in
six
2 3 4 5 6 1
```
###Output example:
```out
five
```
answer：If there is no answer, please comment
The definition of reverse order pair is as follows: for the i-th and j-th elements of a sequence, if I < J and a [i] > a [J], it is a reverse order pair; Otherwise it's not.
###Input format:
The first line contains the integer n, which represents the length of the sequence; The second line contains n integers, representing the entire sequence（ 1≤n≤100000）
###Output format:
Output an integer that represents the number of reverse order pairs.
###Input example:
```in
six
2 3 4 5 6 1
```
###Output example:
```out
five
```
answer：If there is no answer, please comment
