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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;
import java.io.*;

class Main {
    public static void main(String[] args) {
        FastScanner fsc = new FastScanner();
        int n = fsc.nextInt();
        int red[][] = new int[n][2];
        int blue[][] = new int[n][2];
        for (int i = 0; i < n; i++) {
            red[i][0] = fsc.nextInt();
            red[i][1] = fsc.nextInt();
        }
        for (int i = 0; i < n; i++) {
            blue[i][0] = fsc.nextInt();
            blue[i][1] = fsc.nextInt();
        }
        fsc.close();
        Node[] nodes = new Node[2 * n + 2];
        for (int i = 0; i < 2 * n + 2; i++) {
            nodes[i] = new Node(i);
        }
        for (int i = 1; i <= n; i++) {
            addEdge(0, i, 1, nodes);
        }
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if (red[i][0] < blue[j][0] && red[i][1] < blue[j][1]) {
                    addEdge(i + 1, n + j + 1, 1, nodes);
                }
            }
        }
        for (int i = 1; i <= n; i++) {
            addEdge(n + i, 2 * n + 1, 1, nodes);
        }
        long ans = fordFulkerson(0, 2 * n + 1, 2 * n + 2, nodes);
        System.out.println(ans);
    }

    static void addEdge(int fr, int to, int cap, Node[] nodes) {
        nodes[fr].edges.add(new Edge(to, cap, nodes[to].edges.size()));
        nodes[to].edges.add(new Edge(fr, 0, nodes[fr].edges.size() - 1));
    }

    static long fordFulkerson(int s, int t, int n, Node[] nodes) {
        long flow = 0;
        boolean[] used;
        while (true) {
            used = new boolean[n];
            long f = dfs(s, t, nodes, Long.MAX_VALUE, used);
            flow+=f;
            if(f<=0) break;
        }
        return flow;
    }

    private static long dfs(int now, int t, Node[] nodes, long f, boolean[] used) {
        if (now == t) return f;
        used[now]=true;
        for (int i = 0; i < nodes[now].edges.size(); i++) {
            Edge e = nodes[now].edges.get(i);
            if (!used[e.to] && e.cap > 0) {
                long d = dfs(e.to, t, nodes, Math.min(f, e.cap), used);
                if (d > 0) {
                    e.cap -= d;
                    nodes[e.to].edges.get(e.rev).cap += d;
                    return d;
                }
            }
        }
        return 0;
    }

    static class Node {
        int id;
        ArrayList<Edge> edges;

        Node(int id) {
            this.id = id;
            edges = new ArrayList<>();
        }

        @Override
        public boolean equals(Object o) {
            if (!(o instanceof Node))
                return false;
            Node n = (Node) o;
            if (n.id == id)
                return true;
            else
                return false;
        }

        @Override
        public int hashCode() {
            return id;
        }
    }

    static class Edge {
        int to;
        long cap;
        int rev;

        Edge(int to, long cap, int rev) {
            this.to = to;
            this.cap = cap;
            this.rev = rev;
        }
    }

    static class IntQueue {
        private int[] a;
        private int head = 0, tail = 0;
        private int size = 0;

        public IntQueue(int max) {
            a = new int[max];
        }

        public void add(int x) {
            a[tail++] = x;
            size++;
        }

        public int poll() {
            if (size == 0) {
                try {
                    throw new Exception("no elements.");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            size--;
            return a[head++];
        }

        public int size() {
            return size;
        }

        public void init() {
            head = 0;
            tail = 0;
            size = 0;
        }
    }

    // static void addEdge(int fr, int to, int cap, Node[] nodes) {
    //     nodes[fr].edges.add(new Edge(to, cap, nodes[to].edges.size()));
    //     nodes[to].edges.add(new Edge(fr, 0, nodes[fr].edges.size() - 1));
    // }

    // static long dinic(int s, int t, int n, Node[] nodes) {
    //     long flow = 0;
    //     int[] level, iter;
    //     while (true) {
    //         level = bfs(s, t, n, nodes);
    //         if (level[t] < 0) {
    //             break;
    //         }
    //         iter = new int[n];
    //         long f = dfs(s, t, nodes, Long.MAX_VALUE, level, iter);
    //         while (f > 0) {
    //             flow += f;
    //             f = dfs(s, t, nodes, Long.MAX_VALUE, level, iter);
    //         }
    //     }
    //     return flow;
    // }

    // private static int[] bfs(int s, int t, int n, Node[] nodes) {
    //     IntQueue iq = new IntQueue(n);
    //     iq.add(s);
    //     int[] level = new int[n];
    //     for (int i = 0; i < n; i++) {
    //         level[i] = -1;
    //     }
    //     level[s] = 0;
    //     while (iq.size() > 0) {
    //         int now = iq.poll();
    //         for (Edge e : nodes[now].edges) {
    //             if (level[e.to] < 0 && e.cap > 0) {
    //                 iq.add(e.to);
    //                 level[e.to] = level[now] + 1;
    //             }
    //         }
    //     }
    //     return level;
    // }

    // private static long dfs(int now, int t, Node[] nodes, long f, int[] level, int[] iter) {
    //     if (now == t) return f;
    //     for (int i = iter[now]++; i < nodes[now].edges.size(); i++) {
    //         Edge e = nodes[now].edges.get(i);
    //         if (level[now] < level[e.to] && e.cap > 0) {
    //             long d = dfs(e.to, t, nodes, Math.min(f, e.cap), level, iter);
    //             if (d > 0) {
    //                 e.cap -= d;
    //                 nodes[e.to].edges.get(e.rev).cap += d;
    //                 return d;
    //             }
    //         }
    //     }
    //     return 0;
    // }

    // static class Node {
    //     int id;
    //     ArrayList<Edge> edges;

    //     Node(int id) {
    //         this.id = id;
    //         edges = new ArrayList<>();
    //     }

    //     @Override
    //     public boolean equals(Object o) {
    //         if (!(o instanceof Node))
    //             return false;
    //         Node n = (Node) o;
    //         if (n.id == id)
    //             return true;
    //         else
    //             return false;
    //     }

    //     @Override
    //     public int hashCode() {
    //         return id;
    //     }
    // }

    // static class Edge {
    //     int to;
    //     long cap;
    //     int rev;

    //     Edge(int to, long cap, int rev) {
    //         this.to = to;
    //         this.cap = cap;
    //         this.rev = rev;
    //     }
    // }

    // static class IntQueue {
    //     private int[] a;
    //     private int head = 0, tail = 0;
    //     private int size = 0;

    //     public IntQueue(int max) {
    //         a = new int[max];
    //     }

    //     public void add(int x) {
    //         a[tail++] = x;
    //         size++;
    //     }

    //     public int poll() {
    //         if (size == 0) {
    //             try {
    //                 throw new Exception("no elements.");
    //             } catch (Exception e) {
    //                 e.printStackTrace();
    //             }
    //         }
    //         size--;
    //         return a[head++];
    //     }

    //     public int size() {
    //         return size;
    //     }

    //     public void init() {
    //         head = 0;
    //         tail = 0;
    //         size = 0;
    //     }
    // }

    static class FastScanner {
        private final InputStream in = System.in;
        private final byte[] buffer = new byte[1024];
        private int ptr = 0;
        private int buflen = 0;

        private boolean hasNextByte() {
            if (ptr < buflen)
                return true;
            else {
                ptr = 0;
                try {
                    buflen = in.read(buffer);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                if (buflen <= 0)
                    return false;
            }
            return true;
        }

        private int readByte() {
            if (hasNextByte())
                return buffer[ptr++];
            else
                return -1;
        }

        private static boolean isPrintableChar(int c) {
            return 33 <= c && c <= 126;
        }

        public boolean hasNext() {
            while (hasNextByte() && !isPrintableChar(buffer[ptr]))
                ptr++;
            return hasNextByte();
        }

        public String next() {
            if (!hasNext())
                throw new NoSuchElementException();
            StringBuilder sb = new StringBuilder();
            int b = readByte();
            while (isPrintableChar(b)) {
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }

        public long nextLong() {
            if (!hasNext())
                throw new NoSuchElementException();
            long n = 0;
            boolean minus = false;
            int b = readByte();
            if (b == '-') {
                minus = true;
                b = readByte();
            }
            if (b < '0' || '9' < b)
                throw new NumberFormatException();
            while (true) {
                if ('0' <= b && b <= '9') {
                    n *= 10;
                    n += b - '0';
                } else if (b == -1 || !isPrintableChar(b))
                    return minus ? -n : n;
                else
                    throw new NumberFormatException();
                b = readByte();
            }
        }

        public int nextInt() {
            long nl = nextLong();
            if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
                throw new NumberFormatException();
            return (int) nl;
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public void close() {
            try {
                in.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}