#include<fstream>
#include<algorithm>
using namespace std;
struct casa {int l,d;};
int cmp(casa a,casa b)
{if(a.d==b.d)
return a.l<b.l;
return a.d<b.d;}
int main()
{long long dn,dt,i,n,m,max=0,maxt=0;           
casa v[50005];
ifstream q("orase.in");
ofstream w("orase.out");
q>>m>>n;
for(i=1;i<=n;i++)
  q>>v[i].d>>v[i].l;
sort(v+1,v+n+1,cmp);
dt=v[1].l-v[1].d;
for(i=2;i<=n;i++)
  {max=0;
  dn=v[i].l-v[i].d;
  if(dn<dt)
   max=v[i].d+v[i].l+dt;
  else
   {max=v[i].d+v[i].l+dn;
   dt=dn;}
  if(maxt<max)
  maxt=max;}
w<<maxt;
return 0;}