#include <iostream>
#include <fstream>
#include <algorithm>
#include <cmath>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int n,mx=-999999999,dist,m;
struct oras{
    int x,l;
}v[50003];
bool comp(oras a, oras b)
{
    return (a.x<b.x);
}
int main()
{
    f>>m>>n;
    for(int i=1;i<=n;++i)
    {
        f>>v[i].x>>v[i].l;
    }
    sort(v+1,v+n+1,comp);
    for(int i=2;i<=n;++i)
    {
        mx=max(mx,v[i-1].l-v[i-1].x);
        dist=max(dist,v[i].x+v[i].l+mx);
    }
    g<<dist;
    return 0;
}