Gerald plays the following game. He has a checkered field of size n × n cells, where m various cells are banned. Before the game, he has to put a few chips on some border (but not corner) board cells. Then for n - 1 minutes, Gerald every minute moves each chip into an adjacent cell. He moves each chip from its original edge to the opposite edge. Gerald loses in this game in each of the three cases:

  * At least one of the chips at least once fell to the banned cell. 
  * At least once two chips were on the same cell. 
  * At least once two chips swapped in a minute (for example, if you stand two chips on two opposite border cells of a row with even length, this situation happens in the middle of the row). 



In that case he loses and earns 0 points. When nothing like that happened, he wins and earns the number of points equal to the number of chips he managed to put on the board. Help Gerald earn the most points.

Input

The first line contains two space-separated integers n and m (2 ≤ n ≤ 1000, 0 ≤ m ≤ 105) — the size of the field and the number of banned cells. Next m lines each contain two space-separated integers. Specifically, the i-th of these lines contains numbers xi and yi (1 ≤ xi, yi ≤ n) — the coordinates of the i-th banned cell. All given cells are distinct.

Consider the field rows numbered from top to bottom from 1 to n, and the columns — from left to right from 1 to n.

Output

Print a single integer — the maximum points Gerald can earn in this game.

Examples

Input

3 1
2 2


Output

0


Input

3 0


Output

1


Input

4 3
3 1
3 2
3 3


Output

1

Note

In the first test the answer equals zero as we can't put chips into the corner cells.

In the second sample we can place one chip into either cell (1, 2), or cell (3, 2), or cell (2, 1), or cell (2, 3). We cannot place two chips.

In the third sample we can only place one chip into either cell (2, 1), or cell (2, 4).
/* Codeforces Template */

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

import static java.lang.Math.*;
import static java.util.Arrays.fill;
import static java.util.Arrays.binarySearch;
import static java.util.Arrays.sort;

public class Main {
	
	static long initTime;
	static final Random rnd = new Random(7777L);
	static boolean writeLog = false;
	
	public static void main(String[] args) throws IOException {
		initTime = System.currentTimeMillis();
		try {
			writeLog = "true".equals(System.getProperty("LOCAL_RUN_7777"));
		} catch (SecurityException e) {}
		new Thread(null, new Runnable() {
			public void run() {
				try {
					try {
						if (new File("input.txt").exists())
							System.setIn(new FileInputStream("input.txt"));
					} catch (SecurityException e) {}
					long prevTime = System.currentTimeMillis();
					new Main().run();
					log("Total time: " + (System.currentTimeMillis() - prevTime) + " ms");
					log("Memory status: " + memoryStatus());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
		}, "1", 1L << 24).start(); 
	}

	void run() throws IOException {
		in = new BufferedReader(new InputStreamReader(System.in));
		out = new PrintWriter(System.out);
		solve();
		out.close();
		in.close();
	}
	
	/*************************************************************** 
	 * Solution
	 **************************************************************/

	final int[][] conflicts = {
			{ 1, 4, 7 },
			{ 0, 6, 5 },
			{ 3, 5, 6 },
			{ 2, 7, 4 },
			{ 0, 5, 3 },
			{ 2, 4, 1 },
			{ 1, 7, 2 },
			{ 3, 6, 0 },
	};
	
	void solve() throws IOException  {
		
		int n = nextInt();
		int m = nextInt();

		boolean[] badRow = new boolean [n];
		boolean[] badCol = new boolean [n];
		
		for (int i = 0; i < m; i++) {
			int r = nextInt() - 1;
			int c = nextInt() - 1;
			badRow[r] = true;
			badCol[c] = true;
		}
		
		int answer = 0;
		
		for (int i = 1; i < n / 2; i++) {
			int maxBits = 0;
			lp: for (int mask = 0; mask < (1 << 8); mask++) {
				for (int bit = 0; bit < 8; bit++)
					if (testbit(mask, bit))
						for (int badBit : conflicts[bit])
							if (testbit(mask, badBit))
								continue lp;
				
				int goodBits = 0;
				
				for (int bit = 0; bit < 8; bit++) {
					if (testbit(mask, bit)) {
						if ((bit == 0 || bit == 1) && badCol[i]) continue;
						if ((bit == 2 || bit == 3) && badCol[n - 1 - i]) continue;
						if ((bit == 4 || bit == 5) && badRow[i]) continue;
						if ((bit == 6 || bit == 7) && badRow[n - 1 - i]) continue;
						goodBits++;
					}
				}
				
				maxBits = max(maxBits, goodBits);
			}
			answer += maxBits;
		}
	
		if (n % 2 == 1) {
			int h = n / 2;
			if (!badCol[h] || !badRow[h])
				answer++;
		}
		
		out.println(answer);
	}
	
	boolean testbit(int mask, int bit) {
		return ((mask >> bit) & 1) == 1;
	}

	/*************************************************************** 
	 * Input 
	 **************************************************************/
	BufferedReader in;
	PrintWriter out;
	StringTokenizer st = new StringTokenizer("");
	
	String nextToken() throws IOException {
		while (!st.hasMoreTokens())
			st = new StringTokenizer(in.readLine());
		return st.nextToken();
	}
	
	int nextInt() throws IOException {
		return Integer.parseInt(nextToken());
	}
	
	long nextLong() throws IOException {
		return Long.parseLong(nextToken());
	}
	
	double nextDouble() throws IOException {
		return Double.parseDouble(nextToken());
	}
	
	int[] nextIntArray(int size) throws IOException {
		int[] ret = new int [size];
		for (int i = 0; i < size; i++)
			ret[i] = nextInt();
		return ret;
	}
	
	long[] nextLongArray(int size) throws IOException {
		long[] ret = new long [size];
		for (int i = 0; i < size; i++)
			ret[i] = nextLong();
		return ret;
	}
	
	double[] nextDoubleArray(int size) throws IOException {
		double[] ret = new double [size];
		for (int i = 0; i < size; i++)
			ret[i] = nextDouble();
		return ret;
	}
	
	String nextLine() throws IOException {
		st = new StringTokenizer("");
		return in.readLine();
	}
	
	boolean EOF() throws IOException {
		while (!st.hasMoreTokens()) {
			String s = in.readLine();
			if (s == null)
				return true;
			st = new StringTokenizer(s);
		}
		return false;
	}
	
	/*************************************************************** 
	 * Output 
	 **************************************************************/
	void printRepeat(String s, int count) {
		for (int i = 0; i < count; i++)
			out.print(s);
	}
	
	void printArray(int[] array) {
		if (array == null || array.length == 0)
			return;
		for (int i = 0; i < array.length; i++) {
			if (i > 0) out.print(' ');
			out.print(array[i]);
		}
		out.println();
	}
	
	void printArray(long[] array) {
		if (array == null || array.length == 0)
			return;
		for (int i = 0; i < array.length; i++) {
			if (i > 0) out.print(' ');
			out.print(array[i]);
		}
		out.println();
	}
	
	void printArray(double[] array) {
		if (array == null || array.length == 0)
			return;
		for (int i = 0; i < array.length; i++) {
			if (i > 0) out.print(' ');
			out.print(array[i]);
		}
		out.println();
	}
	
	void printArray(double[] array, String spec) {
		if (array == null || array.length == 0)
			return;
		for (int i = 0; i < array.length; i++) {
			if (i > 0) out.print(' ');
			out.printf(Locale.US, spec, array[i]);
		}
		out.println();
	}
	
	void printArray(Object[] array) {
		if (array == null || array.length == 0)
			return;
		boolean blank = false;
		for (Object x : array) {
			if (blank) out.print(' '); else blank = true;
			out.print(x);
		}
		out.println();
	}
	
	@SuppressWarnings("rawtypes")
	void printCollection(Collection collection) {
		if (collection == null || collection.isEmpty())
			return;
		boolean blank = false;
		for (Object x : collection) {
			if (blank) out.print(' '); else blank = true;
			out.print(x);
		}
		out.println();
	}
	
	/*************************************************************** 
	 * Utility
	 **************************************************************/
	static String memoryStatus() {
		return (Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory() >> 20) + "/" + (Runtime.getRuntime().totalMemory() >> 20) + " MB";
	}
	
	static void checkMemory() {
		System.err.println(memoryStatus());
	}
	
	static long prevTimeStamp = Long.MIN_VALUE;
	
	static void updateTimer() {
		prevTimeStamp = System.currentTimeMillis();
	}
	
	static long elapsedTime() {
		return (System.currentTimeMillis() - prevTimeStamp);
	}
	
	static void checkTimer() {
		System.err.println(elapsedTime() + " ms");
	}
	
	static void chk(boolean f) {
		if (!f) throw new RuntimeException("Assert failed");
	}
	
	static void chk(boolean f, String format, Object ... args) {
		if (!f) throw new RuntimeException(String.format(format, args));
	}
	
	static void log(String format, Object ... args) {
		if (writeLog) System.err.println(String.format(Locale.US, format, args));
	}
	
	static void swap(int[] a, int i, int j) {
		int tmp = a[i];
		a[i] = a[j];
		a[j] = tmp;
	}
	
	static void swap(long[] a, int i, int j) {
		long tmp = a[i];
		a[i] = a[j];
		a[j] = tmp;
	}
	
	static void swap(double[] a, int i, int j) {
		double tmp = a[i];
		a[i] = a[j];
		a[j] = tmp;
	}
	
	static void shuffle(int[] a, int from, int to) {
		for (int i = from; i < to; i++)
			swap(a, i, rnd.nextInt(a.length));
	}
	
	static void shuffle(long[] a, int from, int to) {
		for (int i = from; i < to; i++)
			swap(a, i, rnd.nextInt(a.length));
	}
	
	static void shuffle(double[] a, int from, int to) {
		for (int i = from; i < to; i++)
			swap(a, i, rnd.nextInt(a.length));
	}
	
	static void shuffle(int[] a) {
		if (a == null) return;
		shuffle(a, 0, a.length);
	}
	
	static void shuffle(long[] a) {
		if (a == null) return;
		shuffle(a, 0, a.length);
	}
	
	static void shuffle(double[] a) {
		if (a == null) return;
		shuffle(a, 0, a.length);
	}
	
	
}
