#include <iostream>
#include <fstream>

int main(int argv, char **argc)
{
  std::ifstream fin("orase.in");
  int m, n;
  fin >> m >> n;
  int s1 = -m - 1; int s2 = s1;
  for (int i = 0 ; i < n ; ++i)
  {
    int d, l;
    fin >> d >> l;
    int a = d + l;
    if (a > s1)
      s1 = a;
    int b = l - d;
    if (b > s2)
      s2 = b;
  }
  
  fin.close();
  
  std::ofstream fout("orase.out");
  fout << s1 + s2 << std::endl;
  fout.close();
}