#include <bits/stdc++.h>
using namespace std;

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

int main()
{
    int m, n; fin >> m >> n;
    int d[50005], l[50005];

    for (int i = 1; i <= n; i++)
        fin >> d[i] >> l[i];

    int max_dis = 0;

    for (int i = 1; i < n; i++)
        for (int j = i + 1; j <= n; j++)
        {
            int dis_principala = abs(d[i] - d[j]);
            int dis_strada = l[i] + l[j];
            int dis = dis_principala + dis_strada;

            if (dis > max_dis)
                max_dis = dis;
        }

    fout << max_dis;
}