#include <algorithm>
#include <fstream>

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].first - v[i].second);
    }
    fout << suma;
/* DEBUGGING:
    for(int i = 0; i < n; i++)
    {
        fout << i << ": " << v[i].first << " " << v[i].second << "\n";
    }
*/
    return 0;
}

/* To-do:
   -cum se definesc structurile de date cum trebuie
   -ce prostii are pair?
   -sortare vector
*/