#pragma comment(linker, "/STACK:65777216")
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <set>
#include <stack>
#include <map>
#include <queue>
#include <string>
#include <memory.h>
#include <iterator>
#define mem(A,X) memset(A,X,sizeof(A))
#define memo(A) memset(A,0,sizeof(A))
#define forn(I,B) for (int I=1;I<=(B);I++)
#define forg(H,V) for (int H=first[V];h;h=next[H])
#define rep(I,B) for (int I=0;I<(B);I++)
#define labs(X) (((X)>0)?(X):(-(X)))
#define ropen(X) freopen(X,"r",stdin)
#define wopen(X) freopen(X,"w",stdout)
#define pb push_back
#define mp make_pair
#define all(X) (X).begin(),(X).end()

using namespace std;

typedef pair <int,int> pii;
typedef pair <long long,long long> pll;
typedef double ld;
typedef long long ll;
typedef vector<int> vi;
const int N=1111;
const int INF=111111111;
const double eps=1e-9;
const double pi=3.14159265358979;

struct event{
    int x,y1,y2,type;
    bool operator < (const event & q) const{
        return x<q.x;
    }
} t[N+N];

int n,c,curs,co;
ll ans;
vector <pii> a;
vector<pii>::iterator it;

int main(){
//  ropen("input.txt");
    scanf("%d",&n);
    forn(i,n){
        int x1,y1,x2,y2;
        scanf("%d%d%d%d",&x1,&y1,&x2,&y2);
        t[++c]=(event){x1,y2,y1,1};
        t[++c]=(event){x2,y2,y1,-1};
    }
    sort(t+1,t+c+1);
    forn(i,c){
        ans+=(t[i].x-t[i-1].x)*1ll*curs;
        if (t[i].type==-1){
            for (it=a.begin();it!=a.end();it++)
                if (it->first==t[i].y1 && it->second==1){
                    a.erase(it);
                    break;
                }
            for (it=a.begin();it!=a.end();it++)
                if (it->first==t[i].y2 && it->second==-1){
                    a.erase(it);
                    break;
            }
        } else {
            if (a.size() && a.back().first>=t[i].y1){
                for (it=a.begin();it!=a.end();it++)
                    if (it->first>=t[i].y1){
                        a.insert(it,mp(t[i].y1,1));
                        break;
                    }
            } else a.pb(mp(t[i].y1,1));
            if (a.size() && a.back().first>=t[i].y2){
                for (it=a.begin();it!=a.end();it++)
                    if (it->first>=t[i].y2){
                        a.insert(it,mp(t[i].y2,-1));
                        break;
                    }
            } else a.pb(mp(t[i].y2,-1));
        }
        curs=co=0;
        rep(j,a.size()){
            if (co && j) curs+=a[j].first-a[j-1].first;
            co+=a[j].second;
        }
    }
    printf("%I64d\n",ans);
    return 0;
}