#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>
using namespace std;
ifstream fin ("orase.in");
ofstream fout ("orase.out");
int distanta(pair <int, int> x, pair <int, int> y)
{
    return (y.first - x.first + y.second + x.second);
}
int main()
{
    int n,m;
    fin>>m>>n;
    vector <pair<int, int>>v(n);
    for(i=1;i<=n;i++){
        fin>>v[i].first>>v[i].second;
    }
    sort(v.begin(), v.end());
    int dist_max = distanta(v[0], v[1]), u = 0;
    for (int i = 2; i < n; i++)
    {
        if (distanta(v[i-1], v[i]) > distanta(v[u], v[i]))
        {
            u = i - 1;
        }
        dist_max = max(dist_max, distanta(v[u], v[i]));
    }
    out << dist_max << "\n";
    return 0;
}