#include <iostream>
#include <algorithm>
#include <fstream>
using namespace std;
pair <int,int> v[50000];
int main()
{
    ifstream fin("orase.in");
    ofstream fout("orase.out");
    int n,m,i,maxim,s;
    fin>>m>>n;
    for(i=0;i<n;i++)
        fin>>v[i].first>>v[i].second;
    sort(v,v+n);
    maxim=v[0].second-v[0].first;
    s=0;
    for(i=1;i<n;i++)
    {
        s=max(s,v[i].first+v[i].second+maxim);
        maxim=max(maxim,v[i].second-v[i].first);
    }
    fout<<s;
    return 0;
}