# [Usaco2007 Demo]Asteroids

时间限制：5s 空间限制：64MB

### 题目描述

Bessie wants to navigate her spaceship through a dangerous asteroid field in the shape of an N x N grid (1 <= N <= 500). The grid contains K asteroids (1 <= K <= 10,000), which are conveniently located at the lattice points of the grid. Fortunately, Bessie has a powerful weapon that can vaporize all the asteroids in any given row or column of the grid with a single shot. This weapon is quite expensive, so she wishes to use it sparingly. Given the location of all the asteroids in the field, find the minimum number of shots Bessie needs to fire to eliminate all of the asteroids.

### 输入格式

* Line 1: Two integers N and K, separated by a single space. * Lines 2..K+1: Each line contains two space-separated integers R and C (1 <= R, C <= N) denoting the row and column coordinates of an asteroid, respectively.

### 输出格式

* Line 1: The integer representing the minimum number of times Bessie must shoot.

### 样例输入

3 4 1 1 1 3 2 2 3 2 INPUT DETAILS: The following diagram represents the data, where "X" is an asteroid and "." is empty space: X.X .X. .X.

### 样例输出

2 OUTPUT DETAILS: Bessie may fire across row 1 to destroy the asteroids at (1,1) and (1,3), and then she may fire down column 2 to destroy the asteroids at (2,2) and (3,2).

### 提示

没有写明提示

### 题目来源

Gold