#include<fstream>
using namespace std;
struct dl{
       int d;
       int l;};
dl v[50005];
int n,m;
int main()
{
    ifstream fin("orase.in");
    ofstream fout("orase.out");
    fin>>m>>n;
    int i;
    //dl max;
    //max.d=0;
    //max.l=0;
    int j;
    for(i=1;i<=n;i++)
     { 
      fin>>v[i].d;
      fin>>v[i].l;
      //if(v[i].l>max.l)
        //max.l=v[i].l,max.d=v[i].d;
      }
    long long maxm=0;
    for(i=1;i<=n;i++)
       for(j=i+1;j<=n;j++)
        {
        int a=v[i].d-v[j].d;                
         if(a<0)
         a=-a;
       int b=0;
       b=a+v[i].l+v[j].l;
       if(b>maxm)
         maxm=b;
       }
    fout<<maxm;
    return 0;
}