#include <fstream>
#include <algorithm>
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
int n,m,d,r;
pair <int,int> p[50001];
int main()
{ in>>m>>n;
  for(int i=1;i<=n;i++)
    in>>p[i].first>>p[i].second;
  sort(p+1,p+n+1);
  d=r=p[2].first-p[1].first+p[1].second+p[2].second;
  for(int i=3;i<=n;i++)
  { d=max(d-p[i-1].first+p[i].first+p[i].second-p[i-1].second,p[i-1].second+p[i].second+p[i].first-p[i-1].first);
    r=max(d,r);
  }
  out<<r;
  in.close();
  out.close();
  return 0;
}