#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int i,j,m,n,s,max1=-10000;
struct dist{
int di,li;
}a[50009];

bool cmp(dist a, dist b)
{
    if(a.li==b.li)
      return a.li<b.li;
      else
      return a.di<b.di;
}

int main()
{
   f >> m >> n ;
        for ( i = 1 ; i <= n ; i ++ )
           f >> a[i].di >> a[i].li;

        sort(a+1,a+n+1,cmp);

        for ( i = 1 ; i < n ; i ++ )
           for ( j = i + 1 ; j <= n ; j ++ )
             {
                  s = a[j].di-a[i].di +a[j].li+a[i].li;
                 if ( s > max1 )
                   max1 = s;
             }
        g << s;

    return 0;
}