蒟蒻 小号：DerekTan kai123xin https://mo-yu.mysxl.cn/games

#include<bits/stdc++.h>
#define  int long long
using namespace std;
int tree[1000001],n,q,ranks[1000001];
struct node
{
	int val,id;
}a[1000001];
bool cmp(node x,node y)
{
	if(x.val=y.val)return x.id<y.id;
	return x.val<y.val;
}
int lowbit(int y)
{
	return y&-y;
}
void up(int x,int d)
{
	while(x<=n)
	{
		tree[x]+=d;
		x=x+lowbit(x);
	}
}
int ask(int x)
{
	int sum=0;
	while(x>0)
	{
		sum+=tree[x];
		x=x-lowbit(x);
	}
	return sum;
}
main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);cout.tie(0);
	int n;
	cin>>n;
	for(int i=1;i<=n;i++)
    {
    	cin>>a[i].val;
    	a[i].id=i;
	}
	sort(a+1,a+1+n,cmp);
	for(int i=1;i<=n;i++)
	{
		ranks[a[i].id]=i;
	}
	int ans=0;
	for(int i=1;i<=n;i++)
	{
		up(ranks[i],1);
		ans+=i-ask(ranks[i]);
	}
	cout<<ans;
	return 0;
}