#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin ("orase.in");
ofstream fout ("orase.out");
struct orase {
    int d ;
    int l;
};
int cmp (orase a, orase b) {
    return a.d<b.d;
}
int m,n,i;
orase v[50001];
long long maxim, maxt,dn,dt;
int main () {

    fin>>m>>n;
    for (i=1;i<=n;i++)
        fin>>v[i].d>>v[i].l;

    sort (v+1,v+n+1,cmp);
    dt=v[1].l-v[1].d;
    for(i=2;i<=n;i++){
        maxim=0;
        dn=v[i].l-v[i].d;
        if(dn<dt)
            maxim=v[i].l+v[i].d+dt;
        else{
            maxim=v[i].l+v[i].d+dn;
            dt=dn;
        }if(maxt<maxim)
            maxt=maxim;
    }
    fout<<maxt<<"\n";
    return 0;
}