#include<fstream.h>
#include<stdlib.h>
ifstream f1 ("orase.in");
ofstream f2 ("orase.out");
struct strada {int x,y;};
strada str[50001];
int compar (const void*p, const void*q)
{
 strada a=*(strada*)p,b=*(strada*)q;
 if (a.x>b.x) return 1;
 return 0;
}
int main()
{
 int n,m,i,j,nr,dif;
 f1>>m>>n;
 for (i=1; i<=n; i++) f1>>str[i].x>>str[i].y;
 qsort (str+1,n,sizeof(str[1]),compar);
 dif=-2000000;
 nr= -2000000;
 for (i=2; i<=n; i++)
  {if (str[i-1].y-str[i-1].x>dif) dif=str[i-1].y-str[i-1].x;
  if(str[i].x+str[i].y+dif>nr) nr=str[i].x+str[i].y+dif;
  }
 f2<<nr;
 return 0;
}