import heapq
def dijkstra(graph, start):
    distances = [float('inf') for _ in range(len(graph))]
    distances[start] = 0 
    queue = [(0, start)]
    while queue:
        current_distance, current_node = heapq.heappop(queue)
        if current_distance > distances[current_node]:
            continue
        for neighbor, weight in graph[current_node]:
            distance = current_distance + weight
            if distance < distances[neighbor]:
                distances[neighbor] = distance
                heapq.heappush(queue, (distance, neighbor))
    return distances
from heapq import heappop, heappush, heapify
from bisect import bisect
#from sortedcontainers import SortedList
from collections import deque, defaultdict
from math import floor, ceil, isqrt, comb
from sys import stdin, setrecursionlimit
#setrecursionlimit(10**7)
intin = lambda: int(stdin.readline())
strin = lambda: stdin.readline().rstrip()
listin = lambda: list(map(int, stdin.readline().split()))
tuplein = lambda m: [tuple(map(lambda x: int(x) if x.isdigit() or (len(x) > 1 and x[0] == "-" and x[1:].isdigit()) else x, stdin.readline().split())) for _ in range(m)]
gridin = lambda m: [list(map(int, stdin.readline().split())) for _ in range(m)]
strgridin = lambda h: [stdin.readline().rstrip() for _ in range(h)]
mapin = lambda: map(int, stdin.readline().split())
N, M, P, Y = mapin()
ABC = tuplein(M)
DE = tuplein(P)
graph = [[] for _ in range(N)]
for a, b, c in ABC:
    a -= 1; b -= 1
    graph[a].append((b, c))
    graph[b].append((a, c))
dst = dijkstra(graph, 0)
ans = 0
for d, e in DE:
    d -= 1
    ans = max(ans, (Y - dst[d]) // e)
print(ans)