#include <fstream>
#include <algorithm>
#define NMAX 50000

using namespace std;

typedef long long ll;
pair <int, int> v[50000];

int main()
{
    ifstream fin("orase.in");
    ofstream fout("orase.out");
    int m, n;
    fin >> m >> n;
    for (int i = 0; i < n; i++)
    {
        fin >> v[i].first >> v[i].second;
    }
    sort(v, v + n);
    int l = v[0].second;
    int best = 0;
    for (int i = 1; i < n; i++)
    {
        l += v[i].first - v[i-1].first;
        if (l + v[i].second > best)
        {
            best = l + v[i].second;
        }
        if (v[i].second > l)
        {
            l = v[i].second;
        }
    }
    fout << best;

    return 0;
}