#include <fstream>
#include <algorithm>

using namespace std;

int n, i, m, dist, maxim;
pair<int, int> v[50004];

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

int main(){
    fin>>m>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].first>>v[i].second;
    sort(v+1, v+n+1);
    dist = maxim = v[2].first-v[1].first+v[1].second+v[2].second;
    for(int i =3 ; i <= n ;++ i)
    {
        dist=max(dist-v[i-1].first+v[i].first+v[i].second-v[i-1].second,v[i-1].second + v[i].second + v[i].first - v[i-1].first);
       maxim=max(dist,maxim);
    }
    fout<<maxim;
    return 0;
}