#include <bits/stdc++.h>
using namespace std;
ifstream f ("orase.in");
ofstream g ("orase.out");
long long 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;
}