#include<fstream>
#define MMAX 1000010

using namespace std;

int n, m, hmax[MMAX], mx=-MMAX, sol=-MMAX;

ifstream f("orase.in");
ofstream g("orase.out");

void Citeste()
{
    int i, x, y;
    f>>m>>n;
    for (i=1; i<=n; ++i)
    {
        f>>x>>y;
        hmax[x]=max(hmax[x], y);
    }
}

void Solve()
{
    int i;

    for (i=0; i<=m; ++i)
        if (hmax[i])
        {
            if (sol<hmax[i]+i+mx)
                sol=hmax[i]+mx+i;
            if (hmax[i]-i>mx) mx=hmax[i]-i;
        }
    g<<sol<<"\n";
}

int main()
{
    Citeste();
    Solve();
    f.close();
    g.close();
    return 0;
}