#include<fstream>
#include<algorithm>
using namespace std;

typedef struct lol {
        int x,y;
}troll;

int i,n,rs,aux;
troll a[50005];

int main()
{
  ifstream cin("orase.in");
  ofstream cout("orase.out");

  cin>>n>>n>>a[1].x>>a[1].y;
  aux=a[1].y-a[1].x;
  for(i=2;i<=n;++i)
  {
    cin>>a[i].x>>a[i].y;
    aux=max(aux,a[i].y-a[i].x);
  }

  for(i=1;i<=n;++i)
  rs=max(rs,a[i].x+a[i].y);

  cout<<rs+aux<<'\n';

 return 0;
}