#include <stdio.h>
#include <algorithm>
using namespace std;
struct oras {
    int l,d;
} v[50001],aux;
int cmp (oras a,oras b){
    return a.d<=b.d;
}
int m,n,i,j,maxx,p;
int main() {
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&m,&n);
    for(i=1;i<=n;i++)
        scanf("%d%d",&v[i].d,&v[i].l);
    sort(v+1,v+n+1,cmp);
     
    p=1;maxx=0;
    for(i=2;i<=n;i++){
         if(maxx<v[i].d-v[p].d+v[i].l+v[p].l)
                    maxx= v[i].d-v[p].d+v[i].l+v[p].l;
        if(v[i].l-v[i].d>v[p].l-v[p].d) 
            p=i;
         }
    printf("%d",maxx);
    return 0;
}