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

/*
Note for reading ugly variable names more easily:
kx stands for "small x" i.e. the left side,
nx stands for "large x" i.e. the right side
and ky and ny are similarly the bottom and top.
(Abbreviated in Hungarian, my native language...)
*/

int N;

vector<int> X;
vector<int> Y;

struct Rect
{
    int kx, ky, nx, ny;
};

Rect R[1000];

int main()
{
    ios::sync_with_stdio(false);
    cin >> N;
    set<int> sx;
    set<int> sy;
    for (int i = 0; i < N; i++)
    {
        cin >> R[i].kx >> R[i].ny >> R[i].nx >> R[i].ky;
        sx.insert(R[i].kx); sx.insert(R[i].nx);
        sy.insert(R[i].ky); sy.insert(R[i].ny);
    }
    for (int x : sx)
    {
        X.push_back(x);
    }
    for (int y : sy)
    {
        Y.push_back(y);
    }
    bool M[X.size()-1][Y.size()-1];
    for (int i = 0; i < X.size()-1; i++)
        for (int j = 0; j < Y.size()-1; j++)
            M[i][j] = false;
    long long S = 0;
    for (Rect r : R)
    {
        int ki = (lower_bound(X.begin(), X.end(), r.kx) - X.begin());
        int ni = (lower_bound(X.begin(), X.end(), r.nx) - X.begin());
        int kj = (lower_bound(Y.begin(), Y.end(), r.ky) - Y.begin());
        int nj = (lower_bound(Y.begin(), Y.end(), r.ny) - Y.begin());
        for (int i = ki; i < ni; i++)
        {
            for (int j = kj; j < nj; j++)
            {
                if (!M[i][j])
                {
                    M[i][j] = true;
                    S += ((long long)(X[i+1]-X[i]))*(Y[j+1]-Y[j]);
                }
            }
        }
    }
    cout << S << endl;
}