from heapq import heapify,heappush, heappop
def dijkstra(G, s):
    INF = 10**18
    dist = [[INF,INF] for _ in range(N)]
    dist[s][0] = 0
    pq = [(0, s, 10**18)]
    heapify(pq)
    while pq:
        d, v, c  = heappop(pq)
        if d > dist[v][0]:
            continue
        for u, weight in G[v]:
            nd = d + weight
            if dist[u][0] > nd:
                dist[u][0] = nd
                m = 10**18
                if u in D:
                    m = D[u]
                dist[u][1] = min(dist[u][1],m)
                heappush(pq, (nd, u, min(c,m)))
    return dist

N,M,P,Y = map(int,input().split())
G = [set() for _ in range(N)]
for _ in range(M):
    a,b,c = map(int,input().split())
    a,b = a-1,b-1
    G[a].add((b,c))
    G[b].add((a,c))
D = dict()
for _ in range(P):
    d,e = map(int,input().split())
    D[d-1] = e
L = dijkstra(G,0)
if 0 in D:
    L[0][1] = D[0]
Ans = 0
for c,m in L:
    Ans = max(Ans,(Y-c)//m)
print(Ans)