#include <fstream>
#include <algorithm>
struct semix{
    int d,l;
};
semix v[50001];
bool sortare(semix a,semix b){
    return a.d<b.d;
}
using namespace std;
int main(){
    ifstream f("orase.in");
    ofstream g("orase.out");
    int m,n,max,dist,min;
    f>>m>>n;
    int i;
    for(i=1;i<=n;i++)
        f>>v[i].d>>v[i].l;
    sort(v+1,v+n+1,sortare);
    int r = 1;
    max=-1;
    for(i=2 ; i<=n ; i++)
    {
        dist = v[i].d - v[r].d + v[i].l + v[r].l;
        if(dist > max)
            max = dist;
        if(v[r].l + v[i].d - v[r].d < v[i].l)
            r = i;
    }
    g<<max;
    return 0;
}