link
code
#include<bits/stdc++.h>
using namespace std;
const int N = 100010;
int a[N];
int tp[N];
long long ans;
void merge(int l, int r){
if(l >= r) return;
int mid = l + r >> 1;
merge(l ,mid), merge(mid + 1, r);
int i = l, j = mid + 1, k = 0;
while(i <= mid && j <= r){
if(a[i] <= a[j]){
tp[k++] = a[i++];
}else{
tp[k++] = a[j++];
ans += mid - i + 1;
}
}
while(i <= mid) tp[k++] = a[i++];
while(j <= r) tp[k++] = a[j++];
for(int i = 0; i < k; i++)
a[i + l] = tp[i];
}
int main(){
int n; cin >> n;
for(int i = 1; i<= n; i++) cin >>a[i];
merge(1,n);
cout << ans;
return 0;
}