#include <fstream>

using namespace std;

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

int main()
{
    int n, maxx1 = 0, maxx2 = 0, a, b;
    cin >> n >> n;
    for (int i = 1; i <= n; i ++)
    {
        cin >> a >> b;
        maxx1 = max (maxx1, a + b);
        maxx2 = max (maxx2, b - a);
    }
    cout << maxx1 + maxx2 << '\n';
}