#include<fstream.h>
long M,N,a[50000],b[50000];
void citire(){
ifstream fin("orase.in");
fin>>M>>N;
for (long i=0;i<N;i++)
   fin>>a[i]>>b[i];
fin.close();
}
long maxim(){
long max=-10007;
for (long i=0;i<N-1;i++)
   for (long j=i+1;j<N;j++)
	if (a[i]>a[j])
	    if (max<a[i]-a[j]+b[i]+b[j])
		max=a[i]-a[j]+b[i]+b[j];
	else
	  if (a[i]<=a[j])
	      if (max<a[j]-a[i]+b[i]+b[j])
		  max=a[j]-a[i]+b[i]+b[j];
return max;
}
int main(){
citire();
ofstream fout("orase.out");
fout<<maxim();
fout<<"\n";
fout.close();
return 0;
}