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

using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

int m, n;

vector< pair<int, int> > v;

int main()
{
    fin>>m>>n;

    for (int i=1; i<=n; i++)
    {
        int d, l;
        fin>>d>>l;

        v.push_back(make_pair(d, l));
    }

    sort(v.begin(), v.end());

    int st = v[0].first, l = v[0].second, mx = v[0].second;

    for (int i=1; i < v.size(); i++)
    {
        if (l + v[i].first - st + v[i].second > mx) mx = l + v[i].first - st + v[i].second;

        if (l + v[i].first - st < v[i].second)
        {
            l = v[i].second;
            st = v[i].first;
        }
    }

    fout<<mx;

    return 0;
}