#include <fstream>
#include <vector>
#include <algorithm>

using namespace std;

ifstream cin("orase.in");
ofstream cout("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;
    cin>>m>>n;
    vector < pair<int, int>>v(n);
    for(int i=0;i<n;i++)
    {
        cin>>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]));
    }
    cout << dist_max << "\n";
    return 0;
}