#include <iostream>
#include <fstream>
#include <stdlib.h>
#define Max(a,b) a>b ? a:b
using namespace std;
fstream f;
struct orase
{
    long int dps,ps;
}t[50000];
long int m,n;
int i,j,sumamax,aux;
int dist (orase a,orase b)
{
    return a.dps+b.dps+abs(a.ps-b.ps);
}
int main()
{
    f.open("orase.in",ios::in);
    f>>m>>n;
    for (i=0;i<n;i++)
    {
        f>>t[i].ps>>t[i].dps;
    }
    sumamax=-1;
    for (i=0;i<n-1;i++)
    for (j=i+1;j<n;j++)
    {
        aux=dist(t[i],t[j]);
        if (sumamax<aux) sumamax=aux;
    }
    cout<<sumamax;
    f.close();
    f.open("orase.out",ios::out);
    f<<sumamax;
    f.close();
    return 0;
}