#include <fstream>
#include <iostream>
#include <algorithm>

using namespace std;

ifstream f("orase.in");
ofstream g("orase.out");

int n, m, sol, maxi[1000005];

struct drum
{
    int d, l;
}v[1000005];

bool mycmp(drum a, drum b)
{
    return a.d <= b.d;
}

int main()
{
    f >> m >> n;
    for(int i=1; i<=n; i++) f >> v[i].d >> v[i].l;
    sort(v+1, v+n+1, mycmp);

    maxi[1] = 0;
    for(int i=2; i<=n; i++)
    {   maxi[i] = max(maxi[i-1], v[i-1].l-v[i-1].d);
        sol = max(sol, maxi[i]+v[i].l+v[i].d);
    }
    g << sol << '\n';
    return 0;
}