inversion count

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
using ll=long long;
int merge_sort(vector<int>& v,int l,int r){
// base case
    if(r-l<=1) return 0;
    int m=(l+r)/2;
// recursive
    ll ret=0;
    ret+=merge_sort(v,l,m);
    ret+=merge_sort(v,m,r);
// copy
    static vector<int> tmp;
    tmp.resize(v.size());
    copy(v.begin()+l,v.begin()+r,tmp.begin()+l);
// merge
    int ptr=l,lptr=l,rptr=m;
    while(lptr<m&&rptr<r)
        if(tmp[lptr]<=tmp[rptr]){
            v[ptr++]=tmp[lptr++];
            ret+=rptr-m;
        }else
            v[ptr++]=tmp[rptr++];
    while(lptr<m){
        v[ptr++]=tmp[lptr++];
        ret+=rptr-m;
    }
    while(rptr<r) v[ptr++]=tmp[rptr++];
    return ret;
}
void solve(){
    int n;cin>>n;
    vector<int> v(n);
    for(auto& i:v)
        cin>>i;
    cout<<merge_sort(v,0,n)<<endl;
}
int main(){
    solve();
    return 0;
}