from collections import *
import heapq
from itertools import *
from functools import cache, partial
from pprint import pprint
import sys
from typing import Any, Final

try:
    from icecream import ic
except ImportError:  # Graceful fallback if IceCream isn't installed.
    ic = lambda *a: None if not a else (a[0] if len(a) == 1 else a)  # noqa
debug = partial(print, file=sys.stderr)
dpprint = partial(pprint, stream=sys.stderr)
sys.setrecursionlimit(10**6)
MOD = 998244353

N, M, P, Y = map(int, input().split())

G = [[] for _ in range(N)]

for _ in range(M):
    a, b, c = map(int, input().split())
    a, b = a - 1, b - 1
    G[a].append((c, b))
    G[b].append((c, a))

T = [-1] * N
for _ in range(P):
    d, e = map(int, input().split())
    T[d - 1] = e

s = 0
dist = [float("inf")] * N
dist[s] = 0
used = [False] * N
used[s] = True
queue = []
for e in G[s]:
    heapq.heappush(queue, e)
while queue:
    v_cost, v = heapq.heappop(queue)
    if used[v]:
        continue
    dist[v] = v_cost
    used[v] = True
    for u_cost, u in G[v]:
        if not used[u]:
            heapq.heappush(queue, (v_cost + u_cost, u))

ans = 0
for i in range(N):
    if T[i] == -1:
        continue
    t = T[i]
    dist_ = dist[i]
    ans = max(ans, (Y - dist_) // t)

print(ans)