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