#include<fstream.h>
#include<vector>
#include<algorithm>
int maxim,ans,i,j,n,m;
using namespace std;
vector< pair<int,int> >v(50005);
int main()
{ifstream fin("orase.in");
 ofstream fout("orase.out");
 fin>>m>>n;
   for(i=0;i<=n-1;++i)
	   fin>>v[i].first>>v[i].second;
   v.resize(n);
 sort(v.begin(),v.end());
  for(i=0;i<=n-1;++i)
  {ans=max(ans,maxim+v[i].first+v[i].second);
   maxim=max(maxim,v[i].second-v[i].first);
  }
  fout<<ans<<'\n';
 fin.close();
 fout.close();
 return 0;
}