#include<fstream>
#include<algorithm>
#include<iostream>

using namespace std;
ifstream fin("date.in");
ofstream fout("date.out");
int m,n,i,maxi,j,i1,i2;
pair<int, int> v[50001];

int main()
{
    fin>>m>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    maxi=v[2].first-v[1].first+v[2].second+v[1].second;
    i1=2;
    i2=1;
    for(i=3;i<=n;i++)
        if(maxi<v[i].first-v[i1].first+v[i].second+v[i1].second)
        {
            maxi=v[i].first-v[i-1].first+v[i].second+v[i-1].second;
            i2=i1;
            i1=i;
        }
        else
            if(maxi<v[i].first-v[i2].first+v[i].second+v[i2].second)
            {
                maxi=v[i].first-v[i2].first+v[i].second+v[i2].second;
                i1=i;
            }
    fout<<maxi;
    fin.close();
    fout.close();
    return 0;
}