#include <iostream>
#include <vector>
#include <climits>

struct Edge {
    int from, to, time_in, time_out;
    Edge() : from(0), to(0), time_in(0), time_out(0) {};
    Edge(int _from, int _time_in, int _to, int _time_out) : from(_from), to(_to), time_in(_time_in), time_out(_time_out) {};
};

class Graph {
    const int INF = INT_MAX/2;
    std::vector<Edge> edges;
    std::vector<int> moment;
public:
    Graph(int vertexes) : moment(vertexes, INF) {};
    void AddEdge (int v, int time_in, int u, int time_out) {
        edges.push_back(Edge(v, time_in, u, time_out));
    }
    int shortestDistance (int start, int finish) {
        moment[start] = 0;
        for (int i = 0; i < edges.size(); ++i) {
            for (Edge e : edges) {
                if (moment[e.from] > e.time_in) {
                    continue;
                }
                if (e.time_out < moment[e.to] && moment[e.from] != INF) {
                    moment[e.to] = e.time_out;
                }
            }
        }
        return moment[finish];
    }

};



int main() {
    int n;
    std::cin >> n;
    Graph g(n);
    int start, finish;
    std::cin >> start >> finish;
    int k;
    std::cin >> k;
    for (int i = 0; i < k; ++i) {
        int departure, arrival, time_in, time_out;
        std::cin >> departure >> time_in >> arrival >> time_out;
        g.AddEdge(--departure, time_in, --arrival, time_out);
    }
    std::cout << g.shortestDistance(--start, --finish) << "\n";
    return 0;
}