Untitled

#include <iostream>
#include <vector>
using namespace std;

long long merge(std::vector<int>& a, int start, int mid, int end) {
    int n1 = mid - start + 1;
    int n2 = end - mid;
    std::vector<int> left(n1), right(n2);

    for (int i = 0; i < n1; i++) {
        left[i] = a[start + i];
    }
    for (int j = 0; j < n2; j++) {
        right[j] = a[mid + 1 + j];
    }

    int i = 0, j = 0, k = start;
    long long inversions = 0;

    while (i < n1 && j < n2) {
        if (left[i] <= right[j]) {
            a[k] = left[i];
            i++;
        } else {
            a[k] = right[j];
            j++;
            inversions += n1 - i;
        }
        k++;
    }

    while (i < n1) {
        a[k] = left[i];
        i++;
        k++;
    }

    while (j < n2) {
        a[k] = right[j];
        j++;
        k++;
    }

    return inversions;
}

long long mergeSort(std::vector<int>& a, int start, int end) {
    long long inversions = 0;

    if (start < end) {
        int mid = start + (end - start) / 2;
        inversions += mergeSort(a, start, mid);
        inversions += mergeSort(a, mid + 1, end);
        inversions += merge(a, start, mid, end);
    }

    return inversions;
}

int main() {
    
    int n;

    cin >> n;

    vector<int> a;
       
    for(int i=0;i<n;i++){
        int k;
        cin >> k;
         a.push_back(k);
    }
    
    long long result = mergeSort(a, 0, a.size() - 1);
    std::cout << result << std::endl; 

    return 0;
}