#include <bits/stdc++.h>

using namespace std;

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

const int N_MAX = 50005;

int m, n, d, l, maxi1, maxi2;

int main() {
    fin >> m >> n;
    for (int i = 1; i <= n; i++) {
        fin >> d >> l;
        maxi1 = max (maxi1, d + l);
        maxi2 = max (maxi2, -d + l);
    }
    fout << maxi1 + maxi2 << "\n";
    return 0;
}