#include<iostream>
#include<fstream>
#include<algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
struct coord
{
    int x,y;
};
coord v[50001];
int n,m;
int x,y;
bool comp (const coord nr1, const coord nr2)
{
    if(nr1.x==nr2.x)
        return(nr1.y<nr2.y);
    else
        return (nr1.x<nr2.x);
}
int main()
{;
    f>>m>>n;
    for(int i=0;i<n;i++)
    {
        f>>v[i].x;
        f>>v[i].y;
    }
    sort(v,v+n,comp);
    int x1=v[0].x;
    int y1=v[0].y;
    int longest=-1;
    for (int i=1; i<n; i++)
    {
        int x=v[i].x;
        int y=v[i].y;
        if(x+y+y1-x1>longest)
            longest=y+y1+x-x1;
        if(y>x+y1-x1)
           {
               y1=y;
               x1=x;
           }
    }

    g<<longest<<"\n";
}