#include <fstream>
#include <algorithm>
using namespace std;
ifstream cin ("orase.in");
ofstream cout ("orase.out");
struct ura
{
    int d,l;
};
ura v[50010];
bool cmp(ura a,ura b)
{
    if (a.d<b.d)
        return true;
    else
        return false;
}
int main()
{
    int n,m,i,maxx,maxim,x,p;
    cin>>m>>n;
    for (i=1; i<=n; i++)
        cin>>v[i].d>>v[i].l;
    sort (v+1,v+n+1,cmp);
    maxim=v[1].l-v[1].d;
    p=1;
    maxx=-1000000;
    for (i=2; i<=n; i++)
    {
        x=v[p].l+v[i].l+v[i].d-v[p].d;
        maxx=max(maxx,x);
        x=v[i].l-v[i].d;
        if (x>maxim)
        {
            maxim=x;
            p=i;
        }
    }
    cout<<maxx;
    return 0;
}