#include <iostream>
#include <fstream>
using namespace std;
int modul(int b)
{
    if(b<0)
        b=b*(-1);
    return b;
}
int main()
{
    ifstream in("orase.in");
    ofstream out("orase.out");
    int m,n,j,i,a[100001],Smax=0,x,y;
    in>>m>>n;
    for(i=1;i<=2*n;i++)
       in>>a[i];
    for(i=2;i<=2*n;i=i+2)
    {
        for(j=i+2;j<=2*n;j=j+2)
        if(a[j]+a[i]+modul(a[j-1]-a[i-1])>Smax)
             Smax=a[i]+a[j]+modul(a[j-1]-a[i-1]);
    }
    out<<Smax;
    return 0;
}