#include <stdio.h>
#include <stdlib.h>
#define N 50010
struct distanta{
       int d,l;
};
distanta v[N];

int compara(const void *a,const void *b){
    distanta *aa=(distanta*)a,*bb=(distanta*)b;
    if (aa->d>bb->d)
       return 1;
    if (aa->d<bb->d)
       return -1;
    return 0;
}
int main(){
    int n,m,i,j,k,bestk,best,maxim=0;
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&m,&n);
    for (i=0;i<n;++i)
        scanf("%d%d",&v[i].d,&v[i].l);
    qsort(v,n,sizeof(v[0]),compara);
    j=0;
    for (i=1;i<n;++i){
        best=v[j].l-v[j].d;
        for (k=j+1;k<i;++k)
            if (v[k].l-v[k].d>best){
               best=v[k].l-v[k].d;
               bestk=k;
            }
        if (best>v[j].l-v[j].d)
           j=bestk;
        if (v[i].l+v[i].d+v[j].l-v[j].d>maxim)
           maxim=v[i].l+v[i].d+v[j].l-v[j].d;
    }
    printf("%d\n",maxim);
    return 0;
}