def dijkstra(N, S, matrix):
    valid = [True] * (N + 1)
    weight = [1000000] * (N + 1)
    weight[S] = 0
    for i in range(1, N + 1):
        min_weight = 1000001
        ID_min_weight = -1
        for i in range(1, N + 1):
            if valid[i] and weight[i] < min_weight:
                min_weight = weight[i]
                ID_min_weight = i
        for i in lst[ID_min_weight]:
            if weight[ID_min_weight] <= i[1] and i[2] < weight[i[0]]:
                weight[i[0]] = i[2]
        valid[ID_min_weight] = False
    return weight


n = int(input())
d, v = map(int, input().split())
r = int(input())
lst = [[] for i in range(n + 1)]
for i in range(r):
    v1, t1, v2, t2 = map(int, input().split())
    lst[v1].append([v2, t1, t2])
w = dijkstra(n, d, lst)
if w[v] == 1000000:
    print('-1')
else:
    print(w[v])