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

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

int main()
{
    int m, n; fin >> m >> n;

    int maxi = 0, mini = INT_MAX;

    for (int i = 1; i <= n; i++)
    {
        int d, l;
        fin >> d >> l;

        maxi = max(maxi, d + l);
        mini = min(mini, d - l);
    }
    fout << maxi - mini;
}