/*
 * Lefter Sergiu - 14/12/2023
 */
#include <fstream>

using namespace std;

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

const int INF = 2e9;

int main() {
    int n, m, sc, smax = -INF, maxi1 = -INF, maxi2 = -INF;
    in >> n >> m;
    for (int i = 1; i <= m; ++i) {
        int x, y;
        in >> x >> y;
        maxi1 = max(maxi1, x + y);
        maxi2 = max(maxi2, y - x);
    }
    out << maxi1 + maxi2;
    in.close();
    out.close();
    return 0;
}