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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<bits/stdc++.h>
using namespace std;
int n;
int cnt=0,a[105],b[105],c[105],d[105];
bool vis1[105],vis2[105];
vector<int> ans;
bool check(int i,int j)
{
	if(a[i]<c[j]&&b[i]<d[j])
		return true;
	return false;
}
void dfs(int lvl)
{
	for(int i=0;i<n;i++)
	{
		if(!vis2[i]&&check(lvl,i))
		{
			cnt++;
			vis2[i]=true;
			if(lvl==n-1)
				ans.push_back(cnt);
			else
				dfs(lvl+1);
			cnt--;
			vis2[i]=false;
		}
	}
	ans.push_back(cnt);
}
int main()
{
	memset(vis1,0,sizeof(vis1));
	memset(vis2,0,sizeof(vis2));
	cin>>n;
	for(int i=0;i<n;i++)
		cin>>a[i]>>b[i];
	for(int i=0;i<n;i++)
		cin>>c[i]>>d[i];
	dfs(0);
	sort(ans.begin(),ans.end(),greater<int>());
	if(ans.size())
		cout<<ans[0];
	else
		cout<<0;
	return 0;
}