#include<cstdio>
#include<algorithm>
using namespace std;
struct oras{
    int d;
    int l;
}v[50100];
int n,m,i,j,smax;
FILE *f,*g;
int cmp(oras a,oras b){
    return a.d<b.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+n+1,cmp);
    i=1;
    for(j=2;j<=n;j++){
        if(v[j].d+v[j].l+v[i].l-v[i].d>smax)
            smax=v[j].d+v[j].l+v[i].l-v[i].d;
        if(v[j].d+v[i].l<v[i].d+v[j].l)
            i=j;
    }
    fprintf(g,"%d",smax);
    fclose(f);
    fclose(g);
    return 0;
}