using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using System.Numerics;
namespace Solver
{
class Program
{
const int M = 1000000007;
const double eps = 1e-9;
static int n;
static int[][] a;
static void Main()
{
var sw = new System.IO.StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
var sc = new Scan();
int k;
var d = new int[9][];
for (int i = 0; i < 9; i++)
{
d[i] = new int[] { i % 3 - 1, i / 3 - 1 };
}
sc.Multi(out n, out k);
a = new int[k][];
for (int i = 0; i < k; i++)
{
a[i] = sc.IntArr;
}
int max = 0, min = 0;
for (int i = 0; i < k; i++)
{
for (int j = 0; j < k; j++)
{
for (int o = 0; o < k; o++)
{
for (int q = 0; q < k; q++)
{
for (int l = 0; l < 9; l++)
{
for (int m = 0; m < 9; m++)
{
int p = count(a[i][1] + d[l][0], a[o][0] + d[l][1], a[j][1] + d[m][0], a[q][0] + d[m][1]);
max = Math.Max(max, p);
min = Math.Min(min, p);
}
}
}
}
}
}
sw.WriteLine(Math.Max(max, -min));
sw.Flush();
}
static int count(int x1, int y1, int x2, int y2)
{
if (x1 > x2)
return count(x2, y1, x1, y2);
if (y1 > y2)
return count(x1, y2, x2, y1);
if (x1 <= 0 || x2 > n || y1 <= 0 || y2 > n)
return 0;
int ans = 0;
if ((x1 + y1) % 2 == 0)
{
if ((x2 + y2) % 2 == 0)
{
ans = 1;
}
}
else
{
if ((x2 + y2) % 2 == 1)
ans = -1;
}
foreach (var item in a)
{
if (y1 <= item[0] && item[0] <= y2 && x1 <= item[1] && item[1] <= x2)
{
if ((item[0] + item[1]) % 2 == 0)
ans -= 2;
else
ans += 2;
}
}
return ans;
}
static long pow(long a, long b)
{
if (b == 0)
return 1;
if (b == 1)
return a % M;
long t = pow(a, b / 2);
if ((b & 1) == 0)
return t * t % M;
else
return t * t % M * a % M;
}
}
class Scan
{
public int Int { get { return int.Parse(Console.ReadLine().Trim()); } }
public long Long { get { return long.Parse(Console.ReadLine().Trim()); } }
public string Str { get { return Console.ReadLine().Trim(); } }
public int[] IntArr { get { return Console.ReadLine().Trim().Split().Select(int.Parse).ToArray(); } }
public int[] IntArrWithSep(char sep) { return Console.ReadLine().Trim().Split(sep).Select(int.Parse).ToArray(); }
public long[] LongArr { get { return Console.ReadLine().Trim().Split().Select(long.Parse).ToArray(); } }
public double[] DoubleArr { get { return Console.ReadLine().Split().Select(double.Parse).ToArray(); } }
public string[] StrArr { get { return Console.ReadLine().Trim().Split(); } }
public List<int> IntList { get { return Console.ReadLine().Trim().Split().Select(int.Parse).ToList(); } }
public List<long> LongList { get { return Console.ReadLine().Trim().Split().Select(long.Parse).ToList(); } }
public void Multi(out int a, out int b) { var arr = IntArr; a = arr[0]; b = arr[1]; }
public void Multi(out int a, out int b, out int c) { var arr = IntArr; a = arr[0]; b = arr[1]; c = arr[2]; }
public void Multi(out int a, out int b, out int c, out int d) { var arr = IntArr; a = arr[0]; b = arr[1]; c = arr[2]; d = arr[3]; }
public void Multi(out int a, out string b) { var arr = StrArr; a = int.Parse(arr[0]); b = arr[1]; }
public void Multi(out int a, out int b, out string c) { var arr = StrArr; a = int.Parse(arr[0]); b = int.Parse(arr[1]); c = arr[2]; }
public void Multi(out int a, out char b) { var arr = StrArr; a = int.Parse(arr[0]); b = arr[1][0]; }
public void Multi(out long a, out long b) { var arr = LongArr; a = arr[0]; b = arr[1]; }
public void Multi(out long a, out int b) { var arr = LongArr; a = arr[0]; b = (int)arr[1]; }
public void Multi(out string a, out string b) { var arr = StrArr; a = arr[0]; b = arr[1]; }
}
}