#include<fstream>
using namespace std;
struct dl{
       int d;
       int l;
       friend bool operator<(const dl &q,const dl &w)
       {
         return q.d<w.d;}     
         };
dl v[50005];
int n,m;
int main()
{
   ifstream fin("orase.in");
   fin>>m>>n;
   int i;
   for(i=1;i<=n;i++)
     fin>>v[i].d>>v[i].l; 
   sort(v+1,v+n+1);
   int dmax=-1;
   int max=v[1].l,pm=v[i].d;
   for(i=2;i<=n;i++)
   {
     int c=max+v[i].l+(v[i].d-pm);
     if(c>dmax)
       dmax=c;
     if(v[i].l>(max+v[i].d-pm))
       max=v[i].l,pm=v[i].d;                  
       }
    ofstream fout("orase.out");
    fout<<dmax;
    return 0;
  }