#include<fstream.h>
#include<algorithm>
using namespace std;
int main()
{long long dn,dt,v1[50001],v2[50001],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];
  v1[i]=v1[i]*1000000+v2[i];}
sort(v1+1,v1+n+1);
for(i=1;i<=n;i++)
  {v2[i]=v1[i]%1000000;
  v1[i]=v1[i]/1000000;}
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;}