#include <stdio.h>
#include <algorithm>

#define NMAX 50000

int v[NMAX+1],d[NMAX+1];

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

  int i,n,m,a,b,MAX;
  fscanf(fin,"%d%d",&m,&n);
  for(i=0;i<n;i++){
    fscanf(fin,"%d%d",&a,&b);
    v[a]=b;
  }
  MAX=0;
  for(i=0;i<=m;i++){
    MAX=std::max(d[i-1]+1+v[i],MAX);
    d[i]=std::max(v[i],d[i-1]+1);
  }
  fprintf(fout,"%d",MAX);

  fclose(fin);
  fclose(fout);
  return 0;
}