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

using namespace std;

enum Color
{
	WHITE,
	BLACK
};

void dfs(int x, int y, vector <vector<char>>& field, vector <vector<Color>>& colors)
{
	colors[x][y] = BLACK;
	if (field[x + 1][y] == '#' && colors[x + 1][y] == WHITE)
	{
		dfs(x + 1, y, field, colors);
	}
	if (field[x - 1][y] == '#' && colors[x - 1][y] == WHITE)
	{
		dfs(x - 1, y, field, colors);
	}
	if (field[x][y + 1] == '#' && colors[x][y + 1] == WHITE)
	{
		dfs(x, y + 1, field, colors);
	}
	if (field[x][y - 1] == '#' && colors[x][y - 1] == WHITE)
	{
		dfs(x, y - 1, field, colors);
	}
}

int main()

{

	ifstream in("input.txt");
	ofstream out("output.txt");
	int n, ans = -10000000;
	in >> n;
	vector <vector <char>> field (100 + 2, vector <char> (100 + 2, '@'));
	vector <vector <Color>> colors (100 + 2, vector <Color> (100 + 2, WHITE));
	for (int i = 1; i <= 100; i++)
	{
		for (int j = 1; j <= 100; j++)
		{
			field[i][j] = '.';
		}
	}
	for (int i = 1; i <= n; i++)
	{
		int xS, yS, xE, yE;
		in >> xS >> yS >> xE >> yE;
		for (int j = xS; j <= xE; j++)
		{
			for (int k = yS; k <= yE; k++)
			{
				field[j][k] = '#';
			}
		}
	}
	for (int i = 1; i <= 100; i++)
	{
		for (int j = 1; j <= 100; j++)
		{
			if (field[i][j] == '#' && colors[i][j] == WHITE)
			{
				dfs(i, j, field, colors);
			}
		}
	}
	out << ans;
	out.close();
	in.close();
	return 0;
}