#include <iostream>
#include <fstream>
#include <stdlib.h>
#define Max(a,b) a>b ? a:b
using namespace std;
fstream f;
struct orase
{
    long int dps,ps;
}t[50000],inf,sup;
long int m,n,best[50001];
long int i,j,sumamax,aux;
long int lmax;
int dist (orase a,orase b)
{
    return a.dps+b.dps+abs(a.ps-b.ps);
}

void quickSort(orase arr[], int left, int right) {
      int i = left, j = right;
      orase tmp;
      int pivot = arr[(left + right) / 2].ps;

      /* partition */
      while (i <= j) {
            while (arr[i].ps < pivot)
                  i++;
            while (arr[j].ps > pivot)
                  j--;
            if (i <= j) {
                  tmp = arr[i];
                  arr[i] = arr[j];
                  arr[j] = tmp;
                  i++;
                  j--;
            }
      };

      /* recursion */
      if (left < j)
            quickSort(arr, left, j);
      if (i < right)
            quickSort(arr, i, right);
}
int main()
{
    f.open("orase.in",ios::in);
    f>>m>>n;
    for (i=0;i<n;i++)
    {
        f>>t[i].ps>>t[i].dps;
    }
    quickSort(t,0,n-1);

    best[1]=0;
    best[2]=dist(t[0],t[1]);
    sumamax=best[2];
    for (i=3;i<=n;i++)
    {
        best[i]=Max(best[i-1]-t[i-2].dps+t[i-1].dps+t[i-1].ps-t[i-2].ps, dist(t[i-2],t[i-1]));
        sumamax=Max(sumamax,best[i]);
    }

    cout<<sumamax;
    f.close();
    f.open("orase.out",ios::out);
    f<<sumamax;
    f.close();
    return 0;
}