#include <fstream.h>
#include <values.h>

ifstream fin("orase.in");
ofstream fout("orase.out");
long n,m,d[50000],l[50000];

void citire(){
fin>>m>>n;
for (int i=0;i<n;i++)
   fin>>d[i]>>l[i];
fin.close();
}

long maxim(){
 long max=-MAXLONG,max1=max,poz=0;
  for (long i=0;i<n;i++)
  {
      if (l[i]-d[i]>max) {
	 max=l[i]-d[i];
	 poz=i;}
  }
    for (long j=0;j<poz;j++)
	if (l[j]+d[j]>max1)
	   max1=l[j]+d[j];
    for (long y=poz+1;y<n;y++)
       if (l[y]+d[y]>max1)
	  max1=l[y]+d[y];
return max+max1;
}

int main(){
citire();
fout<<maxim();
fout<<"\n";
fout.close();
return 0;
}