#include <algorithm>
#define DIM 50005
using namespace std;
struct oras {int d,l;} a[DIM];
int n,m,best,maxim;
void read ()
{
    int i;
    scanf ("%d%d",&m,&n);
    for (i=1; i<=n; ++i)
        scanf ("%d%d",&a[i].d,&a[i].l);
}
int cmp (oras a,oras b)
{
    return a.d<b.d;
}
void solve ()
{
    int i;
    best=-DIM;
    maxim=0;
    for (i=1; i<=n; ++i)
    {
        if (a[i].d+a[i].l+maxim>best)
            best=a[i].d+a[i].l+maxim;
        if (a[i].l-a[i].d>maxim)
            maxim=a[i].l-a[i].d;
    }
    printf ("%d",best);
}
int main ()
{
    freopen ("orase.in","r",stdin);
    freopen ("orase.out","w",stdout);
    read ();
    sort (a+1,a+n+1,cmp);
    solve ();
    return 0;
}