#include <fstream>

using namespace std;

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

int L, n, d, l, maxim, maxi;
int main()
{
    f >> L >> n;

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

        maxim = max(maxim, d + l);
        maxi = max(maxi, l - d);
    }
    g << maxim + maxi;
}