#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream in ("orase.in");
ofstream out ("orase.out");
const int nmax = 50005;
struct oras{ int d, l; } v[nmax];
inline bool cmp(oras a, oras b)
{
    return (a.d < b.d);
}
int main()
{
    int n, m, i, sol=-1, dist, lg=-1;
    in >> m >> n;
    for(i=1; i<=n; i++)
        in >> v[i].d >> v[i].l;
    sort(v+1, v+n+1, cmp);
    dist=v[1].d;
    for(i=1; i<=n; i++)
    {
        if(v[i].d - dist + lg + v[i].l > sol) sol=v[i].d - dist + lg + v[i].l;
        if(v[i].l > lg+v[i].d-dist)
        {
            dist=v[i].d;
            lg=v[i].l;
        }
    }
    out << sol;
    in.close();
    out.close();
    return 0;
}