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).
// package Practice.CF333;
import java.io.*;
import java.util.*;

/*
    Author - Ribhav
    MADE ON - 12/21/2018
*/

public class CF334D {
    static FastReader s;
    static PrintWriter out;
    static String INPUT = "";

    public static void main (String[] args) {
        long time = System.currentTimeMillis();
        boolean oj = System.getProperty("ONLINE_JUDGE") != null;
        out = new PrintWriter(System.out);
        s = new FastReader(oj);
        int n = s.nextInt();
        int m = s.nextInt();
        int[] col=new int[n];
        int[] row=new int[n];
        for(int i=0;i<m;i++){
            row[s.nextInt()-1]++;
            col[s.nextInt()-1]++;
        }
        int ans=0;
        for(int i=1;i<n/2;i++){
            if(row[i]==0)
                ans++;
            if(col[i]==0)
                ans++;
            if(row[n-i-1]==0)
                ans++;
            if(col[n-i-1]==0)
                ans++;


        }
        if(n%2==1){
            if((col[n/2]==0)||(row[n/2]==0))
                ans++;
        }
        out.println(ans);

        if (!oj) {
            System.out.println(Arrays.deepToString(new Object[] { System.currentTimeMillis() - time + " ms" }));
        }
        out.flush();
    }

    private static class pair{
        int x;
        int y;
    }

    /*----------------------------------------------------------------------------------------------

        Number Theory

    ------------------------------------------------------------------------------------------------
    */


    //    GCD - Using Euclid theorem.
    private static long gcd(long a, long b){
        if(b == 0){
            return a;
        }

        return gcd(b, a % b);
    }

    //    Extended euclidean algorithm
//    Used to solve equations of the form ax + by = gcd(a,b)
//    return array [d, a, b] such that d = gcd(p, q), ap + bq = d
    static long[] extendedEuclidean(long p, long q) {
        if (q == 0)
            return new long[] { p, 1, 0 };

        long[] vals = extendedEuclidean(q, p % q);
        long d = vals[0];
        long a = vals[2];
        long b = vals[1] - (p / q) * vals[2];
        return new long[] { d, a, b };
    }

    // X ^ y mod p
    static long power(long x, long y, long p) {
        // Initialize result
        long res = 1;

        // Update x if it is more
        // than or equal to p
        x = x % p;

        while (y > 0)
        {
            // If y is odd, multiply x
            // with result
            if((y & 1)==1)
                res = (res * x) % p;

            // y must be even now
            // y = y / 2
            y = y >> 1;
            x = (x * x) % p;
        }
        return res;
    }

    // Returns modulo inverse of a
    // with respect to m using extended
    // Euclid Algorithm. Refer below post for details:
    // https://www.geeksforgeeks.org/multiplicative-inverse-under-modulo-m/
    static long inv(long a, long m)
    {
        long m0 = m, t, q;
        long x0 = 0, x1 = 1;

        if (m == 1)
            return 0;

        // Apply extended Euclid Algorithm
        while (a > 1)
        {
            // q is quotient
            q = a / m;

            t = m;

            // m is remainder now, process
            // same as euclid's algo
            m = a % m;a = t;

            t = x0;

            x0 = x1 - q * x0;

            x1 = t;
        }

        // Make x1 positive
        if (x1 < 0)
            x1 += m0;

        return x1;
    }

    // k is size of num[] and rem[].
    // Returns the smallest number
    // x such that:
    // x % num[0] = rem[0],
    // x % num[1] = rem[1],
    // ..................
    // x % num[k-2] = rem[k-1]
    // Assumption: Numbers in num[] are pairwise
    // coprime (gcd for every pair is 1)
    static long findMinX(long num[], long rem[], long k)
    {
        // Compute product of all numbers
        int prod = 1;
        for (int i = 0; i < k; i++)
            prod *= num[i];

        // Initialize result
        int result = 0;

        // Apply above formula
        for (int i = 0; i < k; i++)
        {
            long pp = prod / num[i];
            result += rem[i] * inv(pp, num[i]) * pp;
        }

        return result % prod;
    }




    /*
     *   BINARY SEARCH ALGORITHMS
     *
     *
     *
     * */

    //  Binary search
    private static int binarySearch(int[] arr, int ele){
        int low = 0;
        int high = arr.length - 1;
        while(low <= high){
            int mid = (low + high)/2;
            if(arr[mid] == ele){
                return mid;
            }else if(ele < arr[mid]){
                high = mid - 1;
            }else{
                low = mid + 1;
            }
        }
        return -1;
    }

//    First occurence using binary search

    private static int binarySearchFirstOccurence(int[] arr, int ele){
        int low = 0;
        int high = arr.length - 1;
        int ans = -1;
        while(low <= high){
            int mid = (low + high)/2;
            if(arr[mid] == ele){
                ans = mid;
                high = mid - 1;
            }else if(ele < arr[mid]){
                high = mid - 1;
            }else{
                low = mid + 1;
            }
        }
        return ans;
    }


//    Last occurenece using binary search

    private static int binarySearchLastOccurence(int[] arr, int ele){
        int low = 0;
        int high = arr.length - 1;
        int ans = -1;
        while(low <= high){
            int mid = (low + high)/2;
            if(arr[mid] == ele){
                ans = mid;
                low = mid + 1;
            }else if(ele < arr[mid]){
                high = mid - 1;
            }else{
                low = mid + 1;
            }
        }
        return ans;
    }
    private static class FastReader {
        InputStream is;

        public FastReader(boolean onlineJudge) {
            is = onlineJudge ? System.in : new ByteArrayInputStream(INPUT.getBytes());
        }

        byte[] inbuf = new byte[1024];
        public int lenbuf = 0, ptrbuf = 0;

        int readByte() {
            if (lenbuf == -1)
                throw new InputMismatchException();
            if (ptrbuf >= lenbuf) {
                ptrbuf = 0;
                try {
                    lenbuf = is.read(inbuf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (lenbuf <= 0)
                    return -1;
            }
            return inbuf[ptrbuf++];
        }

        boolean isSpaceChar(int c) {
            return !(c >= 33 && c <= 126);
        }

        int skip() {
            int b;
            while ((b = readByte()) != -1 && isSpaceChar(b))
                ;
            return b;
        }

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

        char nextChar() {
            return (char) skip();
        }

        String next() {
            int b = skip();
            StringBuilder sb = new StringBuilder();
            while (!(isSpaceChar(b))) { // when nextLine, (isSpaceChar(b) && b != ' ')
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }

        String nextLine() {
            int b = skip();
            StringBuilder sb = new StringBuilder();
            while ((!isSpaceChar(b) || b == ' ')) { // when nextLine, (isSpaceChar(b) && b != ' ')
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }

        char[] next(int n) {
            char[] buf = new char[n];
            int b = skip(), p = 0;
            while (p < n && !(isSpaceChar(b))) {
                buf[p++] = (char) b;
                b = readByte();
            }
            return n == p ? buf : Arrays.copyOf(buf, p);
        }

        int nextInt() {
            int num = 0, b;
            boolean minus = false;
            while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
                ;
            if (b == '-') {
                minus = true;
                b = readByte();
            }

            while (true) {
                if (b >= '0' && b <= '9') {
                    num = num * 10 + (b - '0');
                } else {
                    return minus ? -num : num;
                }
                b = readByte();
            }
        }

        long nextLong() {
            long num = 0;
            int b;
            boolean minus = false;
            while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
                ;
            if (b == '-') {
                minus = true;
                b = readByte();
            }

            while (true) {
                if (b >= '0' && b <= '9') {
                    num = num * 10 + (b - '0');
                } else {
                    return minus ? -num : num;
                }
                b = readByte();
            }
        }

        char[][] nextMatrix(int n, int m) {
            char[][] map = new char[n][];
            for (int i = 0; i < n; i++)
                map[i] = next(m);
            return map;
        }

        int[] nextIntArray(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++)
                a[i] = nextInt();
            return a;
        }

        long[] nextLongArray(int n) {
            long[] a = new long[n];
            for (int i = 0; i < n; i++)
                a[i] = nextLong();
            return a;
        }

        int[][] next2DInt(int n, int m) {
            int[][] arr = new int[n][];
            for (int i = 0; i < n; i++) {
                arr[i] = nextIntArray(m);
            }
            return arr;
        }

        long[][] next2DLong(int n, int m) {
            long[][] arr = new long[n][];
            for (int i = 0; i < n; i++) {
                arr[i] = nextLongArray(m);
            }
            return arr;
        }

        int[] shuffle(int[] arr) {
            Random r = new Random();
            for (int i = 1, j; i < arr.length; i++) {
                j = r.nextInt(i);
                arr[i] = arr[i] ^ arr[j];
                arr[j] = arr[i] ^ arr[j];
                arr[i] = arr[i] ^ arr[j];
            }
            return arr;
        }

        int[] uniq(int[] arr) {
            Arrays.sort(arr);
            int[] rv = new int[arr.length];
            int pos = 0;
            rv[pos++] = arr[0];
            for (int i = 1; i < arr.length; i++) {
                if (arr[i] != arr[i - 1]) {
                    rv[pos++] = arr[i];
                }
            }
            return Arrays.copyOf(rv, pos);
        }
    }
}
