1 #include<bits/stdc++.h>
2 #include <iostream>
3 using namespace std;
4 typedef long long ll;
5 typedef unsigned long long ull;
6 vector <int> A;
7 int n,tem;
8 ll merge_count(vector <int> &a)
9 {
10 int m=a.size();
11 if(m<=1)
12 return 0;
13
14 ll cnt=0;
15 vector <int> b(a.begin(),a.begin()+m/2);
16 vector <int> c(a.begin()+m/2,a.end());
17 cnt+=merge_count(b);
18 cnt+=merge_count(c);
19 int ai=0,bi=0,ci=0;
20 while(ai<m)
21 {
22 if(bi<b.size()&&(ci==c.size()||b[bi]<=c[ci]))
23 {
24 a[ai++]=b[bi++];
25 }
26 else
27 {
28 cnt+=m/2-bi;
29 a[ai++]=c[ci++];
30 }
31 }
32 return cnt;
33
34 }
35 int main()
36 {
37 scanf("%d",&n);
38 for(int i=0;i<n;i++)
39 {
40 scanf("%d",&tem);
41 A.push_back(tem);
42 }
43 printf("%lld\n",merge_count(A));
44 return 0;
45 }
