#include <fstream>
#include <algorithm>
using namespace std;

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

const int nmax = 50000;

struct oras {
    int d;
    int l;
};

oras v[nmax + 1];

bool cmp(const oras &a, const oras &b) {
    return a.d < b.d;
}

int main() {
    int m, n;
    fin >> m >> n;

    for (int i = 1; i <= n; i++) {
        fin >> v[i].d >> v[i].l;
    }

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

    int mx = 0;
    int mn = v[1].d - v[1].l;

    for (int i = 1; i <= n; i++) {
        int val = (v[i].d + v[i].l) - mn;
        mx = max(mx, val);
        mn = min(mn, v[i].d - v[i].l);
    }

    fout << mx;
    return 0;
}