#include <fstream>
#include <algorithm>
#define ll long long
using namespace std;

const int NMAX = 100004;

ifstream cin("orase.in");
ofstream cout("orase.out");

struct hell
{
    int d, l;
}v[NMAX];
bool cmp(hell a, hell b)
{
    if (a.d != b.d)
        return a.d < b.d;
    return a.l < b.l;
}
int main()
{
    int n, m, i, ans = 0;
    cin >> m >> n;
    for (i = 1; i <= n; i++)
        cin >> v[i].d >> v[i].l;
    sort(v + 1, v + n + 1, cmp);
    for (i = 1; i < n; i++)
        ans = max(ans, v[n].d - v[i].d + v[i].l + v[n].l);
    cout << ans;
}