# include <iostream>
# include <fstream>
using namespace std;


int d, l;
int n, m;
int maxi, maxj;


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

    fin >> m >> n;
    fin >> d >> l;
    maxi = d +l;
    maxj = l - d;
    for(int i=1; i<n; i++)

    {
        fin >> d >> l;
        if(d + l > maxi)
        {
            maxi = d +l;
        }
        if(l-d > maxj)
        {

            maxj = l-d;
        }


    }
    fout << maxi + maxj;
    fin.close();
    fout.close();

    return 0;
}