#include <fstream>
#include <algorithm>
using namespace std;
ifstream cin("orase.in");
ofstream cout("orase.out");
struct coord{
int x,y;

};
coord v[50001];
bool sem(const coord a, const coord b){
if(a.x==b.x)
  return (a.y<b.y);
  return (a.x<b.x);

}
int main()
{
    int n,m,x,y,maxx,maxy,best=0,xcur,ycur,sol;
    cin>>n>>m;
    for(int i=1;i<=m;i++)
        cin>>v[i].x>>v[i].y;
    sort(v+1,v+m+1,sem);
    maxx=v[1].x;
    maxy=v[1].y;
    for(int i=2;i<=m;i++)
    {
        xcur=v[i].x;
        ycur=v[i].y;
        sol=maxy+ycur+xcur-maxx;
        if(best<sol)
        {
            best=sol;
        }
        if(ycur>maxy+xcur-maxx)
        {
            maxx=xcur;
            maxy=ycur;
        }

    }
    cout<<best;
    return 0;
}