#include <fstream>
#include <algorithm>
#define DEF 50010

using namespace std;

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

pair < int, int > v[DEF], Max;

int sol, n, m;

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

int main () {
    fin >> m >> n;
    for (int i = 1; i <= n; ++ i) {
        fin >> v[i].first >> v[i].second;
    }

    sort (v + 1, v + n + 1, cmp);

    Max = v[1];

    for (int i = 2; i <= n; ++ i) {
        if (v[i].first == Max.first)
            continue;
        sol = max (sol, Max.second + (v[i].first - Max.first) + v[i].second);
        if (v[i].second > Max.second + (v[i].first - Max.first))
            Max = v[i];
    }

    fout << sol;

    return 0;
}