#include <fstream>
#include <algorithm>

using namespace std;

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

pair < int,int > v[50000];

int main()
{
    int m, n, maxim, suma;
    fin >> m >> n;
    for(int i = 0; i < n; i++)
    {
        fin >> v[i].first >> v[i].second;
    }
    sort(v,v+n);
    suma = 0;
    maxim = v[0].second - v[0].first;
    for(int i = 0; i < n; i++)
    {
        suma = max(suma, v[i].first + v[i].second + maxim);
        maxim = max(maxim, v[i].second - v[i].first);
    }
    fout << suma;
    return 0;
}