#include <stdio.h>

#include <stdlib.h>
typedef struct{
    int x,y;
}oras;

oras v[50000];

int cmp(const void *p, const void *q)
{
    oras *pa=(oras*)p,*qb=(oras*)q;
    oras a=(*pa),b=(*qb);
    if (a.x<b.x)
        return -1;
    if (a.x>b.x)
        return 1;
    return 0;
}
int dist(int p,int q){
    return v[p].y+v[q].y+v[q].x-v[p].x;
}
int main()
{
    FILE *fin,*fout;
    int n,i,m,u;
    fin=fopen("orase.in","r");
    fout=fopen("orase.out","w");
    fscanf(fin,"%d%d",&m, &n);
    for(i=0;i<n;i++)
        fscanf(fin,"%d%d", &v[i].x, &v[i].y);
    qsort(v,n,sizeof(v[0]),cmp);
    u=0;
    int dmax=dist(0,1);
    for(i=2;i<n;i++){
        if(dist(i-1,i)>dist(u,i))
            u=i-1;
        if(dist(u,i)>dmax)
            dmax=dist(u,i);
    }
    fprintf(fout,"%d",dmax);
    return 0;
}