#include <fstream>
#include <algorithm>

using namespace std;

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

const int nmax = 5e4;

struct oras {
    int d, l;
} v[nmax + 1];

bool cmp(oras a, oras b) {
    return a.d < b.d; //daca e adevarat returneaza true, altfel false
}

int main() {
    int m, n, i;
    cin >> m >> n;
    for (i = 1; i <= n; i++) {
        cin >> v[i].d >> v[i].l;
    }
    sort(v + 1, v + 1 + n, cmp);
    int ans = -1;
    int minDL = v[1].d - v[1].l;  // minimul D(j) - L(j) pana acum
    for (i = 2; i <= n; i++) {
        ans = max(ans, v[i].d + v[i].l - minDL);
        minDL = min(minDL, v[i].d - v[i].l);
    }
    cout << ans;
    return 0;
}