#include<stdio.h>
void Merge(int ,int ,int );
void mergeSort(int ,int );

int ch[20000],temp[20000];
int count = 0;

void mergeSort(int lo,int hi) 
{
    if(lo < hi) 
    {

        int mid =( lo + hi ) / 2;

        mergeSort(lo,mid);
        mergeSort(mid + 1,hi);

        Merge(lo,mid,hi);
    }
}

 
void Merge(int lo,int mid,int hi) 
{
    int i = lo;
    int j = mid + 1;
    int x = lo;

    while(i <= mid&&j <= hi) 
    {
        if ( ch[i] > ch[j]) 
        {
            count+=  mid - i + 1;
            temp[x++] = ch[j++];
        } 
        else
        {
            temp[x
 
++] = ch[i++];
        }
    }

    while(i <= mid) temp[x++] = ch[i++];
    while(j <= hi)  temp[x++] = ch[j++];

    for(int k = lo; k <= hi ; k++)
        ch[k]  = temp[k];

}
int main() 
{
    int N;
    scanf("%d",&N);

    for(int i=0; i < N; i++) 
    {
        scanf(
 
"%d",&ch[i]);
    }

    mergeSort(0,N-1);

    printf("%d\n",count);
    return 0;
}

歸並排序求逆序對