AC

#include <iostream>
#include <vector>

using namespace std; long long mergeAndCount(vector& arr, vector& temp, int left, int mid, int right) {

int i = left, j = mid + 1, k = left;
long long inv_count = 0;
while (i <= mid && j <= right) {
    if (arr[i] <= arr[j]) {
        temp[k++] = arr[i++];
    } else {
        temp[k++] = arr[j++];
        inv_count += (mid - i + 1);
    }
}
while (i <= mid) {
    temp[k++] = arr[i++];
}
while (j <= right) {
    temp[k++] = arr[j++];
}
for (i = left; i <= right; ++i) {
    arr[i] = temp[i];
}
return inv_count;

} long long mergeSortAndCount(vector& arr, vector& temp, int left, int right) {

long long inv_count = 0;
if (left < right) {
    int mid = left + (right - left) / 2;
    inv_count += mergeSortAndCount(arr, temp, left, mid);
    inv_count += mergeSortAndCount(arr, temp, mid + 1, right);
    inv_count += mergeAndCount(arr, temp, left, mid, right);
}
return inv_count;

} long long countInversions(vector& arr) {

vector<int> temp(arr.size());
return mergeSortAndCount(arr, temp, 0, arr.size() - 1);

} int main() {

ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
vector<int> arr(n);
for (int i = 0; i < n; ++i) {
    cin >> arr[i];
}
cout << countInversions(arr) << '\n';
return 0;

}