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 Rust (1.15.1) (judge verdict: Accepted)

#![allow(unused_parens)]
#![allow(unused_imports)]
#![allow(non_upper_case_globals)]

#[allow(unused_macros)]
macro_rules! invec {
    ( $ t : ty ) => {{
        let mut s = String::new();
        match std::io::stdin().read_line(&mut s) {
            Ok(0) => Vec::<$t>::new(),
            Ok(n) => s
                .trim()
                .split_whitespace()
                .map(|s| s.parse::<$t>().unwrap())
                .collect::<Vec<$t>>(),
            Err(_) => Vec::<$t>::new(),
        }
    }};
}

#[allow(unused_macros)]
macro_rules! get {
      ($t:ty) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              line.trim().parse::<$t>().unwrap()
          }
      };
      ($($t:ty),*) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              let mut iter = line.split_whitespace();
              (
                  $(iter.next().unwrap().parse::<$t>().unwrap(),)*
              )
          }
      };
      ($t:ty; $n:expr) => {
          (0..$n).map(|_|
              get!($t)
          ).collect::<Vec<_>>()
      };
      ($($t:ty),*; $n:expr) => {
          (0..$n).map(|_|
              get!($($t),*)
          ).collect::<Vec<_>>()
      };
      ($t:ty ;;) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              line.split_whitespace()
                  .map(|t| t.parse::<$t>().unwrap())
                  .collect::<Vec<_>>()
          }
      };
      ($t:ty ;; $n:expr) => {
          (0..$n).map(|_| get!($t ;;)).collect::<Vec<_>>()
      };
}

#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
    ($iter:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($next:expr, [$t:tt]) => {
        {
            let len = read_value!($next, usize);
            (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
        }
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

#[cfg(debug_assertions)]
macro_rules! mydbg {
    //($arg:expr) => (dbg!($arg))
    //($arg:expr) => (println!("{:?}",$arg));
      ($($a:expr),*) => {
          eprintln!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
      }
}
#[cfg(not(debug_assertions))]
macro_rules! mydbg {
    ($($arg:expr),*) => {};
}

macro_rules! echo {
    ($($a:expr),*) => {
          $(println!("{}",$a))*
      }
}

struct SliceDisplay<'a, T: 'a>(&'a [T]);
impl<'a, T: std::fmt::Display + 'a> std::fmt::Display for SliceDisplay<'a, T> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        for item in self.0 {
            write!(f, "{} ", item)?;
        }
        Ok(())
    }
}

use std::cmp::*;
use std::collections::*;
use std::ops::{Add, Div, Mul, Sub};

#[allow(dead_code)]
static INF_I64: i64 = 92233720368547758;
#[allow(dead_code)]
static INF_I32: i32 = 21474836;
#[allow(dead_code)]
static INF_USIZE: usize = 18446744073709551;
#[allow(dead_code)]
static M_O_D: usize = 1000000007;
static PAI: f64 = 3.1415926535897932;

#[derive(Clone, Debug, Copy)]
struct Edge {
    to: usize,
    capacity: i64,
    rev: usize,
}

struct Dinic {
    graph: Vec<Vec<Edge>>,
    level: Vec<Option<usize>>,
    iter: Vec<usize>,
}

impl Dinic {
    pub fn new(size: usize) -> Self {
        let mut g = vec![vec![]; size + 1];
        let l = vec![None; size + 1];
        let iter = vec![0; size + 1];
        Dinic {
            graph: g,
            level: l,
            iter: iter,
        }
    }

    pub fn addEdge(&mut self, from: usize, to: usize, cap: i64) {
        mydbg!(from, to);
        let mut rev = self.graph[to].len();
        self.graph[from].push(Edge {
            to: to,
            capacity: cap,
            rev: rev,
        });
        rev = self.graph[from].len() - 1;
        self.graph[to].push(Edge {
            to: from,
            capacity: 0,
            rev: rev,
        });
    }

    fn bfs(&mut self, s: usize) {
        let mut kouho = VecDeque::new();
        self.level = self.level.iter().map(|x| None).collect::<Vec<_>>();
        kouho.push_back(s);
        self.level[s] = Some(0);
        while !kouho.is_empty() {
            let p = kouho.pop_front().unwrap();
            for i in 0..self.graph[p].len() {
                let e = self.graph[p][i];
                if e.capacity > 0 && self.level[e.to].is_none() {
                    self.level[e.to] = Some(self.level[p].unwrap() + 1);
                    kouho.push_back(e.to);
                }
            }
        }
    }

    fn dfs(&mut self, s: usize, t: usize, f: i64) -> i64 {
        if (s == t) {
            return f;
        }
        let mut l = self.iter[s];
        for i in l..self.graph[s].len() {
            let mut e = self.graph[s][i];
            self.iter[s] += 1;
            let tmp = if self.level[s].is_none() {
                0
            } else {
                self.level[s].unwrap()
            };
            if e.capacity > 0 && self.level[e.to].is_some() && tmp <= self.level[e.to].unwrap() {
                let d = self.dfs(e.to, t, min(f, e.capacity));
                if d > 0 {
                    self.graph[s][i].capacity -= d;
                    self.graph[e.to][e.rev].capacity += d;
                    return d;
                }
            }
        }

        return 0;
    }

    pub fn solve(&mut self, start: usize, goal: usize) -> i64 {
        let mut flow = 0;
        loop {
            self.bfs(start);
            mydbg!(self.level);
            if self.level[goal].is_none() {
                return flow;
            }
            self.iter = self.iter.iter().map(|x| 0).collect::<Vec<_>>();
            let mut f = 0;
            while {
                f = self.dfs(start, goal, INF_I64);
                mydbg!(f);
                if f > 0 {
                    flow += f;
                    true
                } else {
                    false
                }
            } {}
        }
    }
}

fn main() {
    input! {
       n: usize,
       mut a:[(i64,i64);n],
       mut b:[(i64,i64);n],
    }

    let mut ans: usize = 0;
    let mut dinic = Dinic::new(2 * n + 2);
    let start = 2 * n + 1;
    let goal = 2 * n + 2;
    mydbg!(start, goal);
    for i in 0..n {
        let (x, y) = a[i];
        for j in 0..n {
            let (x2, y2) = b[j];
            if x < x2 && y < y2 {
                dinic.addEdge(i, j + n, 1);
            }
        }
    }

    for i in 0..n {
        dinic.addEdge(start, i, 1);
        dinic.addEdge(i + n, goal, 1);
    }

    let mut ans = dinic.solve(start, goal);

    echo!(ans);
}