#include <stdio.h>
#include <algorithm>
#define NMAX 50000
using namespace std;

int n,m;
int v[NMAX+3];
FILE *fin,*fout;

int main()
{
    fin=fopen("orase.in","r");
    fout=fopen("orase.out","w");

    fscanf(fin,"%d %d",&n,&m);
    for(int i=1; i<=m; i++)
    {
        int x,d;
        fscanf(fin,"%d %d",&x,&d);
        v[x]=max(v[x],d);
        //fprintf(fout,"%d %d\n",x,v[x]);
    }
    fclose(fin);

    int lMax=0;
    for(int i=0; i<n; i++)
    {
        //fprintf(fout,"%d ",v[i]);
        if(v[i]!=0)
        {
            for(int j=i+1; j<=n; j++)
            {
                if(v[j]!=0)
                {
                   //fprintf(fout,"%d|",v[j]);
                   int lung=v[i]+j-i+v[j];
                   if(lung>lMax)
                   {
                       lMax=lung;
                   }
                }

            }
        }
        //fprintf(fout,"\n");
    }
    fprintf(fout,"%d",lMax);
    return 0;
}