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: Accepted)

namespace AtCoder.ABC.C092
{
    using System;
    using System.Collections;
    using System.Collections.Generic;
    using System.Linq;
    using System.Numerics;
    using System.IO;

    using AtCoder.ABC.IO;

    public class Quest
    {
        public object Solve(Parser parser)
        {
            int n = parser.Integer();

            var red = parser.Take(n).Select(p => new Red()
            {
                x = p.Integer(),
                y = p.Integer(),
            }).OrderByDescending(r => r.y).ToArray();

            Blue[] blue = parser.Take(n).Select(p => new Blue()
            {
                x = p.Integer(),
                y = p.Integer(),
            }).OrderBy(b => b.x).ToArray();

            var g = new Graph();

            for (int i = 0; i < red.Length; i++)
            {
                g.AddEdge(0, 1 + i, 1);
                for (int j = 0; j < blue.Length; j++)
                {
                    if (IsFriend(red[i], blue[j]))
                    {
                        g.AddEdge(1 + i, 1 + j + red.Length, 1);
                    }
                }
            }

            for (int j = 0; j < blue.Length; j++)
            {
                g.AddEdge(1 + j + red.Length, 1 + red.Length + blue.Length, 1);
            }

            return g.MaximizeFlow(0, 1 + red.Length + blue.Length);
        }
        private static bool IsFriend(Red red, Blue blue)
        {
            return red.x < blue.x && red.y < blue.y;
        }
    }

    class Edge
    {
        public int To;
        public int Capacity;
        public int Reverse;
    }

    class Graph
    {
        private Dictionary<int, List<Edge>> m_edges;

        public Graph()
        {
            m_edges = new Dictionary<int, List<Edge>>();
        }


        public int MaximizeFlow(int s, int t)
        {
            int flow = 0;

            while (true)
            {
                bool[] used = Enumerable.Range(0, m_edges.Count)
                .Select(x => false).ToArray();

                int f = Dfs(s, t, int.MaxValue, used);
                if (f == 0)
                {
                    return flow;
                }
                flow += f;
            }
        }

        public void AddEdge(int from, int to, int cap)
        {
            if (!m_edges.ContainsKey(from))
            {
                m_edges.Add(from, new List<Edge>());
            }

            if (!m_edges.ContainsKey(to))
            {
                m_edges.Add(to, new List<Edge>());
            }

            m_edges[from].Add(new Edge()
            {
                To = to,
                Capacity = cap,
                Reverse = m_edges[to].Count,
            });
            m_edges[to].Add(new Edge()
            {
                To = from,
                Capacity = 0,
                Reverse = m_edges[from].Count - 1,
            });
        }

        private int Dfs(int v, int t, int f, bool[] used)
        {
            if (v == t)
            {
                return f;
            }

            used[v] = true;
            foreach (var edge in m_edges[v])
            {
                if (!used[edge.To] && edge.Capacity > 0)
                {
                    int d = Dfs(edge.To, t, Math.Min(f, edge.Capacity), used);
                    if (d > 0)
                    {
                        edge.Capacity -= d;
                        m_edges[edge.To][edge.Reverse].Capacity += d;
                        return d;
                    }
                }
            }

            return 0;
        }
    }

    class Red
    {
        public int x;
        public int y;
    }

    class Blue
    {
        public int x;
        public int y;
    }

    static class Program
    {
        static void Main(string[] args)
        {
            var i = new Parser(Console.OpenStandardInput());
            var o = new Printer(Console.OpenStandardOutput());
            var got = new Quest().Solve(i);
            o.WriteLine(got);
            o.Flush();
        }
    }
}
namespace AtCoder.ABC.IO
{
    #region IO

    using System;
    using System.IO;
    using System.Text;
    using System.Linq;
    using System.Collections.Generic;
    using System.Globalization;

    public class Parser
    {
        public Parser(Stream stream)
        {
            str = stream;
        }

        public readonly Stream str;
        private readonly byte[] buf = new byte[1024];
        private int len, ptr;
        public bool isEof;

        public bool IsEndOfStream
        {
            get { return isEof; }
        }

        public IEnumerable<Parser> Take(int n)
        {
            return Enumerable.Range(1, n).Select(_ => this);
        }

        private byte read()
        {
            if (isEof) return 0;
            if (ptr < len) return buf[ptr++];
            ptr = 0;
            if ((len = str.Read(buf, 0, 1024)) > 0) return buf[ptr++];
            isEof = true;
            return 0;
        }

        public char Char()
        {
            byte b;
            do b = read(); while ((b < 33 || 126 < b) && !isEof);
            return (char)b;
        }

        public string Scan()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b >= 33 && b <= 126; b = (char)read())
                sb.Append(b);
            return sb.ToString();
        }

        public string ScanLine()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b != '\n'; b = (char)read())
                if (b == 0) break;
                else if (b != '\r') sb.Append(b);
            return sb.ToString();
        }

        public long Long()
        {
            if (isEof) return long.MinValue;
            long ret = 0;
            byte b;
            var ng = false;
            do b = read(); while (b != 0 && b != '-' && (b < '0' || '9' < b));
            if (b == 0) return long.MinValue;
            if (b == '-')
            {
                ng = true;
                b = read();
            }
            for (; ; b = read())
            {
                if (b < '0' || '9' < b)
                    return ng ? -ret : ret;
                ret = ret * 10 + b - '0';
            }
        }

        public int Integer()
        {
            return (isEof) ? int.MinValue : (int)Long();
        }

        public double Double()
        {
            var s = Scan();
            return s != "" ? double.Parse(s, CultureInfo.InvariantCulture) : double.NaN;
        }

        static T[] enumerate<T>(int n, Func<T> f)
        {
            var a = new T[n];
            for (int i = 0; i < n; ++i) a[i] = f();
            return a;
        }

        public char[] Char(int n)
        {
            return enumerate(n, Char);
        }

        public string[] Scan(int n)
        {
            return enumerate(n, Scan);
        }

        public double[] Double(int n)
        {
            return enumerate(n, Double);
        }

        public int[] Integer(int n)
        {
            return enumerate(n, Integer);
        }

        public long[] Long(int n)
        {
            return enumerate(n, Long);
        }
    }

    public class Printer : StreamWriter
    {
        public override IFormatProvider FormatProvider
        {
            get { return CultureInfo.InvariantCulture; }
        }

        public Printer(Stream stream)
            : base(stream, new UTF8Encoding(false, true))
        {
        }

        public Printer(Stream stream, Encoding encoding)
            : base(stream, encoding)
        {
        }

        public void Write<T>(string format, T[] source)
        {
            base.Write(format, source.OfType<object>().ToArray());
        }

        public void WriteLine<T>(string format, T[] source)
        {
            base.WriteLine(format, source.OfType<object>().ToArray());
        }
    }

    #endregion

}