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