#include<fstream>
#include<vector>
#include<algorithm>

using namespace std;
typedef int var;

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

#define mp make_pair
typedef pair<var, var> pii;
vector<pii> P;

int main() {

    var n, m, a, b;

    fin>>n>>m;
    while(m--) {
        fin>>a>>b;
        P.push_back(mp(a, b));
    }

    sort(P.begin(), P.end());

    pii best = P[0];
    var best_d = -1;
    for(var i=1; i<P.size(); i++) {
        pii cur = P[i];
        var delta = cur.first - best.first;
        best_d = max(best_d, cur.second + best.second + delta);
        if(best.second + delta < cur.second) {
            best = cur;
        }
    }

    fout<<best_d;

    return 0;
}