#include <fstream>
#include <algorithm>

using namespace std;

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

int distanta(pair<int, int> a, pair<int, int> b)
{
    return b.first-a.first+a.second+b.second;
}

int main()
{
    pair<int, int> v[1000000];
    int n, m, dc, dmax, i;

    in>>m>>n;

    for(i=0; i<n; ++i)
        in>>v[i].first>>v[i].second;

    sort(v, v+n);

    dc=dmax=distanta(v[0], v[1]);

    for(i=2; i<n; ++i){
        dc+=v[i].first-v[i-1].first+v[i].second-v[i-1].second;
        if(distanta(v[i-1], v[i])>dc)
            dc=distanta(v[i-1], v[i]);
        if(dc>dmax)
            dc=dmax;
    }

    out<<dmax;

    return 0;
}