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

using namespace std;
const int N = 50000;
struct strada{
    int stanga,lung;

};
strada v[N+1];
ifstream fi("orase.in");
ofstream fo("orase.out");
bool cmp(strada a, strada b)
{
    if(a.stanga<b.stanga)
    return true;
    else
    return false;
}
int main()
{
    int i,j,m,n,s1,s2,smax;
    fi>>m>>n;
    for(i=1;i<=n;i++)
    {
        fi>>v[i].stanga>>v[i].lung;
    }
    sort(v+1,v+n+1,cmp);
    for(i=1;i<=n;i++)
    {
        s1=v[i].lung+v[i+1].lung+v[i+1].stanga-v[i].stanga;
        s2=v[i].lung+v[i+2].lung+v[i+2].stanga-v[i].stanga;
      if(s1>s2)
            if(s1>smax)
                smax=s1;
        else
            if(s2>smax)
                smax=s2;
    }
    fo<<smax;
    return 0;
}