F

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <string>
#include <set>
#include <stack>
#include <queue>
#include <deque>
using namespace std;

#define TASK "pathbge1"
struct edge {
    int a, b, tin, tout;
};

const int INF = 1e9 + 7;
int n, m, v;
edge e[10000];
int d[10000];

int main() {

#ifdef _DEBUG
    freopen("debug.in", "r", stdin);
    freopen("debug.out", "w", stdout);
#else
    //freopen(TASK".in", "r", stdin);
    //freopen(TASK".out", "w", stdout);
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif // _DEBUG

    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    int i, k, j, s, f, t1, t2;
    cin >> n >> s >> f >> m;
    for (i = 0; i < m; i++)
        cin >> e[i].a >> e[i].tout >> e[i].b >> e[i].tin;
    for (i = 0; i <= n; i++)
        d[i] = INF;
    d[s] = 0;
    for (i = 0; i < n; i++)
        for (j = 0; j < m; j++)
            if (/*d[e[j].a] < INF && */d[e[j].a] <= e[j].tout)
                d[e[j].b] = min(d[e[j].b], e[j].tin);
    cout << d[f] << " ";
    return 0;

    return 0;
}