#include<fstream.h>
int main()
{int aux,v1[501],v2[501],j,i,n,m,max=0,maxt=0;           //00
ifstream q("orase.in");
ofstream w("orase.out");
q>>m>>n;
for(i=1;i<=n;i++)
  q>>v1[i]>>v2[i];
for(i=1;i<n;i++)
  if(v1[i]>v1[i+1])
   {aux=v1[i];
   v1[i]=v1[i+1];
   v1[i+1]=aux;
   aux=v2[i];
   v2[i]=v2[i+1];
   v2[i+1]=aux;
   i=0;}
for(i=1;i<=n;i++)
  {max=0;
  for(j=1;j<i;j++)
    if(v2[i]+v2[j]+v1[i]-v1[j]>max)
     max=v1[i]+v2[i]+v1[j]-v2[j];
  if(maxt<max)
   maxt=max;}
w<<maxt;
return 0;}