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 Python (3.4.3) (judge verdict: Accepted)

import numpy as np
n = int(input())

red_x = []
red_y = []
blue_x = []
blue_y = []
for ls in [[red_x,red_y], [blue_x,blue_y]]:
    for i in range(n):
        x, y = map(int, input().split())
        ls[0].append(x)
        ls[1].append(y)
        
red_x = np.array(red_x)
red_y = np.array(red_y)
blue_x = np.array(blue_x)
blue_y = np.array(blue_y)


def abc091c(red_x, red_y, blue_x, blue_y, count):
    if len(blue_x) > 0:
        blue_min_index = np.where(blue_x == min(blue_x))[0][0] #blue_xの最小値のindexを求める
        x_threshold = blue_x[blue_min_index] #x軸のペア生成可能しきい値
        y_threshold = blue_y[blue_min_index] #y軸のペア生成可能しきい値

        enable_pair_indexes = []
        for i, (r_x, r_y) in enumerate(zip(red_x, red_y)):
            if r_x < x_threshold and r_y < y_threshold:
                enable_pair_indexes.append(i)

        if len(enable_pair_indexes) > 0:
            enable_pair_index = np.where(red_y == max(red_y[enable_pair_indexes]))[0][0]
            #赤い点削除
            red_x = np.delete(red_x, enable_pair_index)
            red_y = np.delete(red_y, enable_pair_index)
            #カウント増加
            count += 1
            
        #青い点削除
        blue_x = np.delete(blue_x, blue_min_index)
        blue_y = np.delete(blue_y, blue_min_index)
        # print(red_x, red_y, blue_x, blue_y)

        abc091c(red_x, red_y, blue_x, blue_y, count)
    else:
        print(count)

count = 0
abc091c(red_x, red_y, blue_x, blue_y, count)