#include<stdio.h>
#include<stdlib.h>
FILE*f=fopen("orase.in","r");
FILE*g=fopen("orase.out","w");
long n,m,d[50009],l[50009],dmax=-100;
void citire()
{
  int i,j;
  fscanf(f,"%ld %ld",&m,&n);
     for(i=1;i<=n;++i)
  fscanf(f,"%ld %ld",&d[i],&l[i]);
}
void calcul()
{
  int i,j;
  for(i=1;i<n;++i)
     for(j=i+1;j<=n;++j)
        if(abs(d[i]-d[j])+l[i]+l[j]>dmax) dmax=abs(d[i]-d[j])+l[i]+l[j];
  fprintf(g,"%ld",dmax);
}
int main()
{
  citire();
  calcul();
  fclose(f);
  fclose(g);
  return 0;
}