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

using namespace std;

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

int m, n, s, mx;
struct oras {
    int l, d;
};
oras a[50005];
inline bool cmp(oras a, oras b)
{
    return a.d < b.d;
}

int main()
{
    fin >> m >> n;
    for (int i = 1; i <= n; i++)
        fin >> a[i].d >> a[i].l;

    sort(a + 1, a + n + 1, cmp);

    for (int i = 1; i <= n; i++)
    {
        if (s < mx + a[i].d + a[i].l)
            s = mx + a[i].d + a[i].l;
        if (mx < a[i].l - a[i].d)
            mx = a[i].l - a[i].d;
    }
    fout << s;
    return 0;
}