#include <fstream>
#include <algorithm>

using namespace std;

ifstream fin ("orase.in");
ofstream fout ("orase.out");

struct str{
    int poz, lung;
};
str v[1000000];
bool cmp (str a, str b){
    if (a.poz!=b.poz){
        if (a.poz < b.poz){
            return true;
        }
        else
            return false;
    }
    else{
        if (a.lung < b.lung)
            return true;
        else
            return false;
    }
}

int main()
{   int n, m, i, j=1;
    fin >> m >> n;
    for (i=1;i<=n;i++){
        fin >> v[i].poz >> v[i].lung;
    }
    sort (v+1, v+n+1, cmp);
    int s=0, minsum=100000, dmax=0;
    for (i=1;i<=n;i++){
        s=v[i].poz+v[i].lung;
        if (s-minsum>dmax){
            dmax=s-minsum;
        }
        if (minsum>s-2*v[i].lung){
            minsum=s-2*v[i].lung;
        }
    }
    fout << dmax;
    return 0;
}