#include <fstream>
#include <queue>
#include <algorithm>
using namespace std;
ifstream cin("orase.in");
ofstream cout("orase.out");
priority_queue<int> pque;
struct city {
  int d, h;
  bool operator< (const city& x) const {
    return d<x.d;
  }
}v[50000];
int main() {
  int n,maxx;
  cin >> n >> n;
  for(int i=0; i<n; i++)
    cin >> v[i].d >> v[i].h;
  sort(v,v+n);
  pque.push(v[0].h-v[0].d);
  maxx=0;
  for(int i=1; i<n; i++) {
    maxx=max(maxx,pque.top()+v[i].h+v[i].d);
    pque.push(v[i].h-v[i].d);
  }
  cout << maxx << '\n';
  
}