#include <iostream>
#include <queue>
#include <tuple>
#include <vector>
#include <set>

using namespace std;

struct my_pair_g {
    int v;
    int weight;

    my_pair_g(int v_, int weight_) :
            v(v_), weight(weight_) {}
};

struct my_pair_s {
    int64_t dist;
    int v;

    my_pair_s(int64_t dist_, int v_) :
            dist(dist_), v(v_) {}
};

struct timetable {
    int st_time;
    int fin_time;

    timetable(int s_, int f_) :
            st_time(s_), fin_time(f_) {}

    timetable() :
            st_time(0), fin_time(0) {}
};

int main() {
    int N;
    cin >> N;
    int start;
    cin >> start;
    --start;
    int finish;
    cin >> finish;
    --finish;
    int M;
    cin >> M;
    vector<vector<vector<timetable>>> graph(N, vector<vector<timetable>>(N));
    for (int i = 0; i < M; ++i) {
        int a_vil;
        cin >> a_vil;
        --a_vil;
        int st_time;
        cin >> st_time;
        int b_vil;
        cin >> b_vil;
        --b_vil;
        int fin_time;
        cin >> fin_time;
        graph[a_vil][b_vil].emplace_back(st_time, fin_time);
    }
    vector<int64_t> distances(N, 3000'000'000'00);
    distances[start] = 0;
    auto cmp = [](my_pair_s a, my_pair_s b) { return tie(a.dist, a.v) < tie(b.dist, b.v); };
    set<my_pair_s, decltype(cmp)> s_que(cmp);
    s_que.insert({distances[start], start});
    while (!s_que.empty()) {
        int vert = s_que.begin()->v;
        s_que.erase(s_que.begin());
        for (int pot_neig = 0; pot_neig < N; ++pot_neig) {
            if (!graph[vert][pot_neig].empty()) {
                vector<timetable> neig_v = graph[vert][pot_neig];//если есть сосед на этой позиции
                for (size_t j = 0; j < neig_v.size(); ++j) {
                    if (distances[vert] <= neig_v[j].st_time) {
                        if (distances[pot_neig] > neig_v[j].fin_time) {
                            distances[pot_neig] = neig_v[j].fin_time;
                        }
                    }
                }
                s_que.insert({distances[pot_neig], pot_neig});
            }
        }
    }
    if (distances[finish] == 3000'000'000'00) {
        cout << -1;
    }else{
        cout << distances[finish];
    }
}