#include <stdio.h>
#include <stdlib.h>
typedef struct{
    int x,y;
}oras;
oras v[50000];

int cmpfunc (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;
    fin=fopen("orase.in","r");
    fout=fopen("orase.out","w");
    int n,i,m,u;
    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]),cmpfunc);
    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;
}