#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
int main()
{
   int n,m,i,j,mx1=0,a,b,mx2=0;
   fin>>m>>n;
   for(i=1;i<=n;i++)
   {
       fin>>a>>b;
       if(mx1<a+b)
       {
           mx1=a+b;
       }
       if(mx2<b-a)
       {
           mx2=b-a;
       }
   }
  fout<<mx1+mx2;
    return 0;
}