#include <iostream>

#define MMAX 1000000

using namespace std;

int v [ MMAX + 1 ] ;
int d [ MMAX + 1 ] ;

int main() {
  FILE *fin, *fout ;
  fin = fopen ("orase.in", "r" ) ;
  fout = fopen ("orase.out", "w" ) ;
  int n, i, maxim, m, a, b ;
  fscanf (fin, "%d%d", &m, &n ) ;
  maxim = 0 ;
  for (i = 0 ; i < n ; i++ ) {
    fscanf (fin, "%d%d", &a, &b ) ;
    v[a] = max (v[a], b ) ;
  }
  for (i = 0 ; i <= m ; i++ ) {
    if (d[i-1] + 1 + v[i] > maxim )
      maxim = d[i-1] + 1 + v[i] ;
    d[i] = max (d[i-1] + 1, v[i] ) ;
  }
  fprintf (fout, "%d", maxim ) ;
  return 0;
}