#include <fstream>
#include <algorithm>
using namespace std;
int n,m;
pair <int,int> v[50005];
int main()
{
    ifstream f ("orase.in");
    ofstream g ("orase.out");
    f>>m>>n;
    for (int i=1; i<=n; i++)
        f>>v[i].first>>v[i].second;
    sort (v+1,v+n+1);
    int rsp=0,mx=0;
    for (int i=1; i<=n; i++)
    {
        rsp=max(rsp,mx+v[i].first+v[i].second);
        mx=max(mx,v[i].second-v[i].first);
    }
    g<<rsp;
}