#include <fstream>
#include <algorithm>

using namespace std;

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

pair < int, int > v[50001];

bool cmp(pair < int, int > a, pair < int, int > b)
{
    if (a.first < b.first)
        return 1;
    return 0;
}

int main()
{
    int n, m, i, j, MAX = 0;
    in >> m >> n;
    for (i = 1; i <= n; ++i)
        in >> v[i].first >> v[i].second;
    sort(v + 1, v + n + 1, cmp);
    for (i = 1; i <= n; ++i)
    {
        for (j = 1; j <= i; ++j)
        {
            if (v[i].second + v[j].second + v[i].first - v[j].first > MAX)
                MAX = v[i].second + v[j].second + v[i].first - v[j].first;
        }
    }
    out << MAX;
    return 0;
}