#include <stdio.h>
#include <algorithm>
using namespace std;

int m, n, i, j, k;
long int dist, Max;

struct strazi
{
    int d;
    int l;
};

struct strazi v[50001];

int abs (int x)
{
    if (x < 0)
        return x * -1;
    return x;
}

int calculare_distanta ()
{
    return (v[i].l + v[j].l + abs (v[i].d - v[j].d));
}

int main ()
{
    FILE *f = fopen ("orase.in","r");
    FILE *g = fopen ("orase.out","w");
    fscanf (f,"%d %d", &m, &n);
    for (i=1; i<=n; ++i)
        fscanf (f,"%d %d", &v[i].d, &v[i].l);

    for (i=1; i<n; ++i)
        for (j=i+1; j<=n; ++j)
        {
            dist = calculare_distanta ();
            if (dist > Max)
                Max = dist;
        }

    fprintf (g, "%ld\n", Max);

    fclose (g);
    fclose (f);
    return 0;
}