关于本题的ans计数的疑惑?

#include <bits/stdc++.h>
using namespace std;
/*     本次的代码中，数组要从1开始计数，不从0开始        */
long long int a[500050],tool[500050],ans=0;

int main()
{
    void mer_sort(long long int a[],long long int lef,long long int mid,long long int rig);
    void merge_tool(long long int a[],long long int lef,long long int rig);
    long long int n;    scanf("%lld", &n);
    for(long long int i=1;i<=n;i++) scanf("%lld", &a[i]);
    merge_tool(a,1,n);
    printf("%lld",ans);
    system("pause");
}

void mer_sort(long long int a[],long long int lef,long long int mid,long long int rig)
{
    long long int i=lef,j=mid+1,k=lef;//lef , mid & rig need attention!!!
    while(i<=mid&&j<=rig)
    {
        if(a[i]<=a[j]) tool[k++]=a[i++];
        else
        {           
            tool[k++]=a[j++];           
            ans+=mid-i+1;
        }       
    }
    if(i>mid)
        for(long long int t=j;t<=rig;t++)   tool[k++]=a[t];
    if(j>rig)
        for(long long int t=i;t<=mid;t++)   tool[k++]=a[t];
    for(long long int t=lef;t<=rig;t++) a[t]=tool[t];
}

void merge_tool(long long int a[],long long int lef,long long int rig)
{
    if(lef>=rig) return;
    else
    {
        merge_tool(a,lef,(lef+rig)/2);
        merge_tool(a,(rig+lef)/2+1,rig);
        mer_sort(a,lef,(rig+lef)/2,rig);
    }
}

就是在mer_sort()函数中的27行
ans的计数代码那里
为何要使用ans+=mid-i+1；
鄙人用ans+=j-i; 就全WA了 /雾