#include <bits/stdc++.h>
#include <fstream>

using namespace std;

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

long long int m , n , d , l , i , maxx1 , maxx2 , rez;

int main()
{
    f >> m;

    f >> n;

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

        maxx1 = max ( maxx1 , d + l );
        maxx2 = max ( maxx2 , l - d );
    }

    rez = maxx1 + maxx2;

    g << rez << " ";

    return 0;
}