#include <iostream>
#include <cmath>
#include <fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
int main() {
    int n, m, i, max1, max2, x, y;
    fin>>n>>m;

    for(i=1; i<=m; i++) {
        fin>>x>>y;
        while(max1<x+y)
        {
            max1=x+y;
        }

        while(max2<y-x)
        {
            max2=y-x;
        }
    }
    fout<<max1+max2;
    return 0;
}