#include <fstream>
#include <algorithm>
using namespace std;
pair <int, int> v[1000005];
int distanta(pair <int, int> a, pair <int, int> b)
{
    return b.first - a.first + a.second + b.second;
}
int main()
{
    ifstream cin("orase.in");
    ofstream cout("orase.out");
    int m,n;
    cin>>m>>n;
    for(int i=0;i<n;i++)
        cin>>v[i].first >>v[i].second;
    sort(v,v+n);
    int dmax=0,dc=0;
    dmax=dc=distanta(v[0],v[1]);
    for(int 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]);
        dmax=max(dmax,dc);
    }
    cout<<dmax;
    return 0;
}