#include<fstream>
using namespace std;
struct dl{
       int d;
       int l;};
dl v[50005];
int n,m;
int main()
{
    ifstream fin("orase.in");
    ofstream fout("orase.out");
    fin>>m>>n;
    int i;
    dl max;
    max.d=0;
    max.l=0;
    for(i=1;i<=n;i++)
     { 
      fin>>v[i].d;
      fin>>v[i].l;
      if(v[i].l>max.l)
        max.l=v[i].l,max.d=v[i].d;
      }
    long long maxm=0;
    for(i=1;i<=n;i++)
     {
       int a=max.d-v[i].d;                
       if(a<0)
         a=-a;
       int b=0;
       b=a+max.l+v[i].l;
       if(b>maxm)
         maxm=b;
       }
    fout<<maxm;
    return 0;
}