在一个排列中，如果一对数的前后位置与大小顺序相反，即前面的数大于后面的数，那么它们就称为一个逆序。一个排列中逆序的总数就称为这个排列的逆序数。

现在，给你一个N个元素的序列，请你判断出它的逆序数。-In one arrangement, if the front and rear position and size of a pair of numbers in reverse order, that is greater than the number in front of the back number, then they are called a reverse. The total number of a reverse arrangement is called the number listed in reverse order.

Now, to give you a sequence of N elements, you determine that it s the inverse number.