#include<fstream.h>
int main()
{int dn,dt,key,key2,aux,v1[50001],v2[50001],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=2;i<=n;i++)
  {key=v1[i];
  key2=v2[i];
  j=i-1;
  while(j>0&&v1[j]>key)
   {v1[j+1]=v1[j];
   v2[j+1]=v2[j];
   j--;}
   v1[j+1]=key;
   v2[j+1]=key2;}
dt=v2[1]-v1[1];
for(i=2;i<=n;i++)
  {max=0;
  dn=v2[i]-v1[i];
  if(dn<dt)
   max=v2[i]+v1[i]+dt;
  else
   {max=v2[i]+v1[i]+dn;
   dt=dn;}
  if(maxt<max)
   maxt=max;}
w<<maxt;
return 0;}