Problem:
Score : 400 points

Problem Statement

On a two-dimensional plane, there are N red points and N blue points.
The coordinates of the i-th red point are (a_i, b_i), and the coordinates of the i-th blue point are (c_i, d_i).

A red point and a blue point can form a friendly pair when, the x-coordinate of the red point is smaller than that of the blue point, and the y-coordinate of the red point is also smaller than that of the blue point.

At most how many friendly pairs can you form? Note that a point cannot belong to multiple pairs.

Constraints

All input values are integers.

1 \leq N \leq 100

0 \leq a_i, b_i, c_i, d_i < 2N

a_1, a_2, ..., a_N, c_1, c_2, ..., c_N are all different.

b_1, b_2, ..., b_N, d_1, d_2, ..., d_N are all different.

Input

Input is given from Standard Input in the following format:

N
a_1 b_1
a_2 b_2
:
a_N b_N
c_1 d_1
c_2 d_2
:
c_N d_N

Output

Print the maximum number of friendly pairs.

Sample Input 1

3
2 0
3 1
1 3
4 2
0 4
5 5

Sample Output 1

2

For example, you can pair (2, 0) and (4, 2), then (3, 1) and (5, 5).

Sample Input 2

3
0 0
1 1
5 2
2 3
3 4
4 5

Sample Output 2

2

For example, you can pair (0, 0) and (2, 3), then (1, 1) and (3, 4).

Sample Input 3

2
2 2
3 3
0 0
1 1

Sample Output 3

0

It is possible that no pair can be formed.

Sample Input 4

5
0 0
7 3
2 2
4 8
1 6
8 5
6 9
5 4
9 1
3 7

Sample Output 4

5

Sample Input 5

5
0 0
1 1
5 5
6 6
7 7
2 2
3 3
4 4
8 8
9 9

Sample Output 5

4

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections;
using System.Linq;
class Program
{
    public static void Main()
    {
        int n = int.Parse(Console.ReadLine());
        int[,] red = new int[n, 2];
        int[,] blue = new int[n, 2];
        string[] str;
        for (int i = 0; i < n; i++)
        {
            str = Console.ReadLine().Split(' ');
            red[i, 0] = int.Parse(str[0]);
            red[i, 1] = int.Parse(str[1]);
        }
        for (int i = 0; i < n; i++)
        {
            str = Console.ReadLine().Split(' ');
            blue[i, 0] = int.Parse(str[0]);
            blue[i, 1] = int.Parse(str[1]);
        }
        bool[,] po = new bool[n, n];
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < n; j++)
            {
                po[i, j] = (red[i, 0] < blue[j, 0]) && (red[i, 1] < blue[j, 1]);
            }
        }
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (po[i, j])
                {
                    bool f1 = false;
                    bool f2 = false;
                    for (int p = 0; p < n; p++)
                    {
                        if (!(p == i))
                            f1 = f1 || po[p, j];
                    }
                    for (int p = 0; p < n; p++)
                    {
                        if (!(p == j))
                            f2 = f2 || po[i, p];
                    }
                    if (f1 && f2)
                        po[i, j] = false;
                }
            }
        }
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (po[i, j])
                {
                    bool f1 = false;
                    bool f2 = false;
                    for (int p = 0; p < n; p++)
                    {
                        if (!(p == i))
                            f1 = f1 || po[p, j];
                    }
                    for (int p = 0; p < n; p++)
                    {
                        if (!(p == j))
                            f2 = f2 || po[i, p];
                    }
                    if (f1 || f2)
                        po[i, j] = false;
                }
            }
        }
        int ans = 0;
        for (int i = 0; i < n; i++)
            for (int j = 0; j < n; j++)
                if (po[i, j])
                    ans++;
        Console.WriteLine(ans);
    }
}