import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

from heapq import *
INF = 10**18
def dijkstra(g, s, n):
    dist = [INF]*n
    hq = [(0, s)]
    dist[s] = 0
    while hq:
        d, v = heappop(hq)
        if dist[v]!=d:
            continue
        for to, c in g[v]:
            if dist[v] + c < dist[to]:
                dist[to] = dist[v] + c
                heappush(hq, (dist[to], to))
    return dist
n, m, p, y = na()
g = [[] for i in range(n)]
for _ in range(m):
    u, v, c = na()
    u -= 1
    v -= 1
    g[u].append((v, c))
    g[v].append((u, c))

dist = dijkstra(g, 0, n)
d, e = zip(*[na() for i in range(p)])
d = [i-1 for i in d]
ans = 0
for i in range(p):
    ans = max(ans, (y - dist[d[i]]) // e[i])
print(ans)