#include<fstream>
#include<cmath>
#include<stdlib.h>
#define Nmax 1000001
using namespace std;

int n,m,V[Nmax],sum;
int L[Nmax],D[Nmax];
int Smax;
ifstream f("orase.in");
ofstream g("orase.out");

int main()
{
    f>>n>>m;
 int i,j;
  for(i=1;i<=m;++i)
   f>>D[i]>>L[i];


for(i=1;i<=m;++i)
{
    for(j=i+1;j<=m;++j)
   {
       sum=L[i]+L[j]+abs(D[i]-D[j]);
       if(sum>Smax)
       Smax=sum;
   }
}
g<<Smax;

f.close();
g.close();
return 0;
}