#include <fstream>
#include <algorithm>

using namespace std;

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

const int N = 50001;

struct oras
{
    int d, l;
};

oras v[N];

int distanta(oras x, oras y)
{
    return y.d - x.d + y.l + x.l;
}

bool cmp(oras x, oras y)
{
    return (x.d < y.d);
}

int main()
{
    int m, n;
    fin >> m >> n;
    for(int i = 0; i < n; i++)
    {
        fin >> v[i].d >> v[i].l;
    }
    sort(v, v + n, cmp);
    int dist_max = distanta(v[0], v[1]), u = 0;
    for(int i = 2; i < n; i++)
    {
        if(distanta(v[i - 1], v[i]) > distanta(v[u], v[i]))
        {
            u = i - 1;
        }
        if(distanta(v[u], v[i]) > dist_max)
        {
            dist_max = distanta(v[u], v[i]);
        }
    }
    fout << dist_max;
    return 0;
}