#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(){
short ok=0;
long p=N;
while (ok==0){
ok=1;
p--;
for (long i=0;i<p;i++)
    if (a[i]<a[i+1]){
       long aux=a[i];
       a[i]=a[i+1];
       a[i+1]=aux;
       ok=0;
       aux=b[i];
       b[i]=b[i+1];
       b[i+1]=aux;}
}}
long maxim(){
long max=-10007;
for (long i=0;i<N-1;i++)
   for (long j=i+1;j<N;j++)
     if ((b[i]+b[j]+a[i]-a[j])>max)
	max=b[i]+b[j]+a[i]-a[j];
return max;
}
int main(){
citire();
bule();
ofstream fout("orase.out");
fout<<maxim();
fout<<"\n";
fout.close();
return 0;
}