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

using namespace std;

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

vector<pair<int, int>> v;
int best, n, m, d, l, maxx;

int main()
{
    fin >> m >> n;
    for (; n; n--){
        fin >> d >> l;
        v.push_back(make_pair(d, l));
    }

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

    for (auto it: v){
        maxx = max(maxx, it.first + it.second + best);
        best = max(best, it.second - it.first);
    }

    fout << maxx;
    return 0;
}