#include <fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
int main(){
  int n, i, maxst, maxdr, a, b;
  fin >> n >> n;
  maxst = maxdr = 0;
  for (i = 0; i < n; i++){
    fin >> a >> b;
    maxst = max(maxst, a + b);
    maxdr = max(maxdr, b - a);
  }
  fout << maxst + maxdr << "\n";
  fin.close();
  fout.close();
  return 0;
}