#define _USE_MATH_DEFINES
#include <string>
#include <stack>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <fstream>
#include <deque>
#include <vector>
#include <stdlib.h>
#include <set>
#include <queue>
#include <iostream>
#include <climits>


using namespace std;


int dfs(pair<int, int> v, vector <vector<char>>& mas, vector <vector<char>>& colors) {
    if (mas[v.first][v.second] == '.' || colors[v.first][v.second] == 'B') {
        return 0;
    }

    colors[v.first][v.second] = 'B';

    return dfs(make_pair(v.first - 1, v.second), mas, colors) +
        dfs(make_pair(v.first + 1, v.second), mas, colors) +
        dfs(make_pair(v.first, v.second - 1), mas, colors) +
        dfs(make_pair(v.first, v.second + 1), mas, colors) + 1;
}

int main() {

    ifstream in("input.txt");
    ofstream out("output.TXT");

    vector<vector<char>> vec(102, vector<char>(102, '.'));
    vector<vector<char>> colors(102, vector<char>(102, 'W'));

    int n;
    in >> n;

    for (int i = 0; i < n; i++) {
        int x1, y1, x2, y2;
        in >> x1 >> y1 >> x2 >> y2;
        for (int u = y1; u < y2; u++) {
            for (int v = x1; v < x2; v++) {
                vec[u + 1][v + 1] = '#';
            }
        }
    }

    int maxS = 0;
    for (int i = 1; i <= 100; i++) {
        for (int j = 1; j <= 100; j++) {
            if (vec[i][j] == '#' && colors[i][j] != 'B') {
                maxS = max(maxS, dfs(make_pair(i, j), vec, colors));
            }
        }
    }

    out << maxS;

    //for (int i = 0; i < vec.size(); i++) {
    //  for (int j = 0; j < vec.size(); j++) {
    //      out << vec[i][j] << ' ';
    //  }
    //  out << endl;
    //}

    out.close();
    in.close();
    return 0;
}