english-32 Reverse Pair

描述

Given a sequence S of N numbers. How many pairs of indexes I and J are there satisfying that I is less than J and S[i] is greater than S[J]?

输入

First line: one positive integers N (N <= 100).Second line: N positive integers (<= 1000).

输出

One integer.

样例输入

1
25
4 2 5 3 1

样例输出

17

限制

• 时间限制：1000ms
• 内存限制：128MB