#include <fstream>
#include <iostream>
#include <algorithm>

#define NMAX 50003

using namespace std;

 ifstream fin ("orase.in");
 ofstream fout ("orase.out");

int n,m,i,hmax,s,mx;

struct oras{
    int d,l;
}v[NMAX];

int cmp(oras a,oras b)
{
    return a.d<b.d;
}

int main()
{

    fin>>m>>n;
    for(i=1;i<=n;i++)
    {
        fin>>v[i].d>>v[i].l;
    }
    sort(v+1,v+n+1,cmp);
    for(i=1;i<=n;i++)
    {
        hmax=max(hmax,mx+v[i].d+v[i].l);
        mx=max(mx,v[i].l-v[i].d);
    }
    fout<<hmax;
    return 0;
}