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

const int MAX_LENGTH = 50002;

struct City{
    int position, length;
};

bool sortCity(City first, City second){
    if (first.position < second.position){
        return true;
    }else if (first.position == second.position && first.length < second.length){
        return true;
    }
    return false;
}

int main (void){
    ifstream fin ("orase.in");

    int length, toIgnore;
    fin>>toIgnore>>length;

    City city[MAX_LENGTH];

    for (int i=1; i<=length; i++){
        fin>>city[i].position>>city[i].length;
    }
    fin.close();

    sort(city+1, city+length+1, sortCity);

    int maxPath = city[1].length;
    int maxPathIndex = 1;

    int maxim = 0;

    for (int i=2; i<=length; i++){
        int aux = maxPath + city[i].position - city[maxPathIndex].position + city[i].length;
        if (aux > maxim)
            maxim = aux;

        if (city[i].length > (maxPath + city[i].position - city[maxPathIndex].position)){
            maxPath = city[i].length;
            maxPathIndex = i;
        }
    }

    ofstream fout("orase.out");
    fout<<maxim<<"\n";
    fout.close();

    return 0;
}