#include<cstdio>
#include<algorithm>
using namespace std;
struct date{
    int d;
    int l;
};
date v[50001];
int n,m,i,j,a,b,s,smax;
FILE *f,*g;
int cmp(date x,date y){
    return x.d<y.d;
}
int main(){
    f=fopen("orase.in","r");
    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);
    }
    sort(v+1,v+1+n,cmp);
    smax=0;
    j=1;
    for(i=2;i<=n;i++){
        s=v[i].l+v[i].d+v[j].l-v[j].d;
        if(s>smax)
            smax=s;
        if(v[i].d+v[j].l<v[i].l+v[j].d)
            j=i;
    }
    fprintf(g,"%d",smax);
    fclose(f);
    fclose(g);
    return 0;
}