#include <iostream>
#include <cmath>
#include <fstream>
#include <algorithm>
using namespace std;
pair<int, int> v[50005];
int n, m, maxim, lung;
int main() {
    cin >> m >> n;
    for (int i = 1; i <= n; i ++) {
        cin >> v[i].first >> v[i].second;
    }
    sort(v+1, v+n+1);
    int i = 1;
    for (int j = 2; j <= n; j ++) {
        int lung = v[i].second + (v[j].first - v[i].first) + v[j].second;
        if (lung > maxim)
            maxim = lung;
        if (v[j].second > v[i].second + (v[j].first - v[i].first)) {
            i = j;
        }
    }
    cout << maxim;
    return 0;
}