#出典:https://atcoder.jp/contests/abc362/submissions/55346606
from heapq import *
def dijkstra(G, s):
    INF = 10**18
    dist = [INF] * len(G)
    dist[s] = 0
    pq = [(0, s)]
    while pq:
        d, v = heappop(pq)
        if d > dist[v]:
            continue
        for u, weight in G[v]:
            nd = d + weight
            if dist[u] > nd:
                dist[u] = nd
                heappush(pq, (nd, u))
    return dist
N,M,P,Y=map(int,input().split())
g=[[] for i in range(N)]
for i in range(M):
    A,B,C=map(int,input().split())
    g[A-1].append([B-1,C])
    g[B-1].append([A-1,C])
shops=[10**18 for i in range(N)]
for i in range(P):
    D,E=map(int,input().split())
    shops[D-1]=E
dist=dijkstra(g,0)
#print(dist)
#print(shops)
ans=0
for i in range(N):
    #各街について、所持金はY-dist[i]
    #print(Y-dist[i],shops[i])
    #print((Y-dist[i])//shops[i])
    ans=max(ans,(Y-dist[i])//shops[i])
print(ans)