#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
pair <int,int>v[60000],best;
int n,i;
unsigned long long maxx;
int main()
{
    fin>>n>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    best=v[1];
    for(i=2;i<=n;i++){
        if(v[i].first-best.first+best.second+v[i].second>maxx){
            maxx = v[i].first - best.first + best.second + v[i].second;
        }
        if(v[i].first - best.first + best.second < v[i].second)
            best=v[i];

    }
    fout<<maxx;
}