#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>

using namespace std;

typedef long long ll;

const ll INF = 2000000000;

struct Rec {
    ll x1, y1, x2, y2;

    Rec(ll x1 = 0, ll y1 = 0, ll x2 = 0, ll y2 = 0) : x1(x1), y1(y1), x2(x2), y2(y2) {
    }
};

struct Event {
    int type; //start 0, end 1
    int id;
    ll x;

    Event(int id, int type, ll x) : id(id), type(type), x(x) {
    }
};

struct EventCmp {

    bool operator()(const Event& a, const Event& b) {
        return (a.x < b.x) || (a.x == b.x && a.type > b.type);
    }
};

struct IntervalCmp {
    vector<Rec> recs;

    IntervalCmp(vector<Rec>& recs) : recs(recs) {
    }

    bool operator()(const int& a, const int& b) {
        return recs[a].y1 < recs[b].y1;
    }
};

ll intervalLength(vector<int>& intervals, vector<Rec>& recs, vector<bool>& flags) {
    ll start = -INF, end;
    ll total = 0;


    for (int idx : intervals) {
        if (flags[idx]) {
            Rec& cur = recs[idx];
            if (start == -INF) {
                start = cur.y1;
                end = cur.y2;
            }
            else if (cur.y1 <= end) {
                end = max(end, cur.y2);
            } else {
                total += end - start;
                start = cur.y1;
                end = cur.y2;
            }
            //cout << "s/e " << start << " " << end << endl;
        }
    }
    if (start != -INF) {
        total += end - start;
    }
    return total;
}

int main() {
    int n;
    cin >> n;
    vector<Rec> recs(n);
    vector<Event> events;

    for (int i = 0; i < n; i++) {
        Rec& rec = recs[i];
        cin >> rec.x1 >> rec.y1 >> rec.x2 >> rec.y2;
        events.push_back(Event(i, 0, rec.x1));
        events.push_back(Event(i, 1, rec.x2));
    }

    vector<int> intervals(n);
    for (int i = 0; i < n; i++) {
        intervals[i] = i;
    }

    sort(events.begin(), events.end(), EventCmp());
    sort(intervals.begin(), intervals.end(), IntervalCmp(recs));

    //    for (int i = 0; i < n; i++) {
    //        cout << recs[i].x1 << " " << recs[i].y1 << " " << recs[i].x2 << " " << recs[i].y2 << endl;
    //    }

    int lastX = 0;
    ll lastLen = 0;
    ll area = 0;

    vector<bool> flags(n, false);
    for (Event event : events) {
        //cout << "event " << event.type << " " << event.id << " " << event.x << endl;
        area += (event.x - lastX) * lastLen;
        lastX = event.x;
        if (event.type == 0) {
            flags[event.id] = true;
        } else {
            flags[event.id] = false;
        }
        lastLen = intervalLength(intervals, recs, flags);
    }
    cout << area << endl;
}