/*
ID: SkyHawk
PROG: planting
LANG: C++
*/

#include <stdio.h>
#include <iostream>
#include <list>
#include <string>
#include <vector>
#include <iterator>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <set>
#include <list>
#include <queue>

using namespace std;

#define FOR(i,n) for(int i = 0;i < n;++i)
#define PII pair<int,int>

vector<pair<PII ,PII > > rect;

int used[21];

int main(int argc, char** argv)
{
    freopen("planting.in","r",stdin);
    freopen("planting.out","w",stdout);
    int n;
    cin >> n;
    FOR(i,n)
    {
        int x1,y1,x2,y2;
        cin >> x1 >> y1 >> x2 >> y2;
        rect.push_back(make_pair(PII(x1,0), PII(y1,y2)));
        rect.push_back(make_pair(PII(x2,1), PII(y1,y2)));
    }
    sort(rect.begin(),rect.end());
    int h=0,x=-10001;
    long long s = 0;
    FOR(i,2*n)
    {
        if(rect[i].first.second == 0)
        {
            s+=h*(rect[i].first.first-x);
            used[i] = 1;
            h = 0;
            for(int j = -10001;j<=10001;++j)
                FOR(k,n)
                {
                    if((used[k]) && (j>=rect[k].second.second &&
                        j<rect[k].second.first))
                    {
                        h++;
                        break;
                    }
                }
            x = rect[i].first.first;
        }
        else
        {
            s+=h*(rect[i].first.first-x);
            FOR(j,i)
                if(used[j] && rect[j].second == rect[i].second)
                {
                    used[j] = 0;
                    break;
                }
            h = 0;
            for(int j = -10001;j<=10001;++j)
                FOR(k,n)
                {
                    if((used[k]) && (j>=rect[k].second.second &&
                        j<rect[k].second.first))
                    {
                        h++;
                        break;
                    }
                }
            x = rect[i].first.first;

        }

    }
    cout << s<< endl;
    return 0;
}