#include<fstream.h>
long M,N,a[50001],b[50001];
void citire(){
ifstream fin("orase.in");
fin>>M>>N;
for (long i=0;i<N;i++)
   fin>>a[i]>>b[i];
fin.close();
}
void bule (long N){
short ok=0;
while (ok==0){
ok=1;
N--;
for (long i=0;i<N;i++)
    if (a[i]>a[i+1]){
      long aux=a[i];
      a[i]=a[i+1];
      a[i+1]=aux;
      aux=b[i];
      b[i]=b[i+1];
      b[i+1]=aux;
      ok=0;}
}}
long maxim(){
long max=-10007,max2=-1010101;
   for (long j=0;j<N;j++)
      if (b[j]-a[j]>max)
	max=b[j]-a[j];
   for (long i=0;i<N;i++)
       if (a[i]+b[j]>max2)
	 max2=a[i]+b[i];
return max+max2;
}
int main(){
citire();
bule(N);
ofstream fout("orase.out");
fout<<maxim();
fout<<"\n";
fout.close();
return 0;
}