#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;

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

const int dmax = 50000;

struct ELEMENT {int d, L;};
ELEMENT m[dmax + 1];

int Dist_max;

int M, N;

bool exc(ELEMENT e1, ELEMENT e2)
{
    if(e1.d == e2.d) return e1.L < e2.L;
    else
        return e1.d < e2.d;
}

int main()
{
    int i, j, dist;

    in >> M >> N;

    for(i = 1; i <= N; i++) in >> m[i].d >> m[i].L;

    sort(m + 1, m + N + 1, exc);

    for(i = 1; i <= N; i++)
        for(j = i + 1; j <= N; j++)
        {
            if(m[j].d == m[i].d) dist = m[j].L - m[i].L;
            else
                dist = m[i].L + ( m[j].d - m[i].d ) + m[j].L;

            Dist_max = max(Dist_max, dist);
        }

    out << Dist_max;

    return 0;
}