#include <fstream>

using namespace std;

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

int m,n,i,l,d;
int maxim1,maxim2,rez;

int main()
{
    f>>m>>n;
    for (i=1;i<=n;i++)
    {
        f>>d>>l;
        maxim1=max(maxim1,l+d);
        maxim2=max(maxim2,l-d);
    }
    rez=maxim1+maxim2;
    g<<rez;
    return 0;
}