#include <algorithm>
#include <iostream>
#include <fstream>
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
const int maxn = 50005;
pair <int, int> v[maxn];

int main()
{
    int n, m;
    in >> m >> n;
    for(int i = 1; i <= n; i++)
        in >> v[i].first >> v[i].second;
    sort(v + 1, v + n + 1);
    int mx = v[1].second - v[1].first;
    int lgmax = 0;
    for(int i = 2; i <= n; i++)
    {
        lgmax = max(lgmax, mx + v[i].second + v[i].first);
        mx = max(mx, v[i].second - v[i].first);
    }
    out << lgmax << "\n";
    return 0;
}