#include <iostream.h>
#include <fstream.h>

#define NMAX 50000

int main(void)
{
 fstream f1("orase.in",ios::in);
 fstream f2("orase.out",ios::out);
 long M = 0;
 long N = 0;
 long a = 0;
 long b = 0;
 f1 >> M >> N;
 long Strazi[NMAX][2];
 long DMAX = 0;
 long D = 0;
 for (a = 0;a < N;a = a + 1)
  {
   f1 >> Strazi[a][0] >> Strazi[a][1];
//           D                 L
  }
 for (a = 0;a < N;a = a + 1)
  {
   for (b = (a + 1);b < N;b = b + 1)
    {
     if (Strazi[b][0] > Strazi[a][0])
       {
        D = Strazi[a][1] + Strazi[b][1] - Strazi[a][0] + Strazi[b][0];
       }
      else
       {
        D = Strazi[a][1] + Strazi[b][1] + Strazi[a][0] - Strazi[b][0];
       }
     if (D > DMAX)
       {
        DMAX = D;
       }
    }
  }
 f2 << DMAX; 
 f1.close();
 f2.close();
 return 0;
}