#include <fstream>
#include<algorithm>
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
struct pct
{
    int x, y ;
};
pct v[50001];
int n, m,i;
bool cmp(pct a , pct b)
{
    if(a.x<b.x)
        return 1;
    return 0;
}
int distanta(int i , int j)
{
    return v[i].x-v[j].x+v[i].y+v[j].y;
}
int main()
{
    in>>m>>n;
    for(i=1;i<=n;i++)
        in>>v[i].x>>v[i].y;
    sort(v+1,v+1+n,cmp);
    int u=1,d,dmax=0;
    for(i=1;i<=n;i++)
    {
        d=distanta(i,u);
        if(d>dmax)
            dmax=d;
        if(v[i].x-v[u].x+v[u].y<v[i].y)
            u=i;

    }
    out<<dmax;
}