#include <fstream>
#include <algorithm>

using namespace std;

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

int main()
{
    int n, m, i, a, b, MAX1 = 0, MAX2 = 0;
    in >> m >> n;
    for (i = 1; i <= n; ++i)
    {
        in >> a >> b;
        MAX1 = max(MAX1, a + b);
        MAX2 = max(MAX2, b - a);
    }
    out << MAX1 + MAX2;
    return 0;
}