#include <fstream>
#include <algorithm>
using namespace std;
    ifstream fin("orase.in");
    ofstream fout("orase.out");
long int n, m, i, j, minim, maxim;
pair <long int , long int> v[50001];
int main(){
    fin >> m >> n;
    for(i = 1; i <= n; i ++)
        fin >> v[i].first >> v[i].second;
    sort(v + 1, v + n + 1);
    minim = v[1].second - v[1].first;
    j = 1;
    for(i = 2; i <= n; i ++){
        if(maxim < v[i].second + v[j].second + (v[i].first - v[j].first))
            maxim = v[i].second + v[j].second + (v[i].first - v[j].first);
        if(minim < v[i].second - v[i].first){
            minim = v[i].second - v[i].first;
            j = i;
        }
    }
    fout << maxim;
    return 0;
}