Inversions

#include <bits/stdc++.h>

using namespace std;

int calculateAnswer(vector<int> &p, int l, int r) {
    if (l == r) {
        return 0;
    }
    int m = (l + r) / 2;
    int answer = calculateAnswer(p, l, m) + calculateAnswer(p, m + 1, r);

    int i = l, j = m;
    for (; i <= m; i++) {
        while (j + 1 <= r && p[i] > p[j + 1]) {
            j++;
        }
        answer += j - m;
    }

    vector<int> v;
    i = l, j = m + 1;
    while (i <= m && j <= r) {
        if (p[i] < p[j]) {
            v.push_back(p[i++]);
        } else {
            v.push_back(p[j++]);
        }
    }
    while (i <= m) {
        v.push_back(p[i++]);
    }
    while (j <= r) {
        v.push_back(p[j++]);
    }
    for (i = l; i <= r; i++) {
        p[i] = v[i - l];
    }

    return answer;
}

int main() {
    int n;
    cin >> n;

    vector<int> p(n);
    for (auto &i : p) {
        cin >> i;
    }

    int answer = calculateAnswer(p, 0, n - 1);
    cout << answer;
}