import collections,sys,math,functools,operator,itertools,bisect,heapq,decimal,string,time,random
#sys.setrecursionlimit(10**9)
#sys.set_int_max_str_digits(0)
input = sys.stdin.readline
#n = int(input())
#alist = list(map(int,input().split()))
#alist = []
#s = input()
n,m,p,y= map(int,input().split())
#for i in range(n):
#    alist.append(list(map(int,input().split())))
edge = [[] for i in range(n)]
for i in range(m):
    u,v,c = map(int,input().split())
    u-=1
    v-=1
    edge[u].append((v,c))
    edge[v].append((u,c))
z = [10**18 for i in range(n)]
for i in range(p):
    d,e = map(int,input().split())
    z[d-1] = e
start = 0
d = []
dist = [10**18 for i in range(n)]
heapq.heappush(d,(0,0))
dist[start] = 0
while d:
    _,now = heapq.heappop(d)
    if _ > dist[now]:
        continue
    for to,cost in edge[now]:
        if dist[to] > dist[now] + cost:
            dist[to] = dist[now] + cost
            heapq.heappush(d,(dist[to],to))
ans = 0
for i in range(n):
    ans = max(ans,(y-dist[i])//z[i])
print(ans)