#include <fstream>

using namespace std;

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

struct oras
{
    int l, d;
};

int main()
{
    int n, m, mx1 =0, mx2=-1000000, i1, i2 = -1, s;
    oras a[50001];

    fin >> m >> n;

    for (int i = 1; i<=n; i++)
    {
        fin >> a[i].d >> a[i].l;
        if (a[i].d + a[i].l > mx1)
        {
            i1 = i;
            mx1 = a[i].d + a[i].l;
        }
        else if (a[i].l - a[i].d > mx2)
        {
            mx2 = a[i].l - a[i].d;
            i2 = i;
        }
    }
    if (i2 < i1 && i2 != -1)s = mx2 + mx1 + 2 * a[i2].d - 2 * a[i1].d;
    else s = mx1 + mx2;

    fout << s;
    return 0;
}