from collections import defaultdict
import heapq
N,M,P,Y=list(map(int,input().split()))
dist=[10**18]*N
path=defaultdict(list)
for i in range(M):
	u,v,c=list(map(int,input().split()))
	u-=1
	v-=1
	path[u].append((v,c))
	path[v].append((u,c))
dist[0]=0
c=set()
Q=[]
heapq.heapify(Q)
heapq.heappush(Q,(0,0))
while len(Q)>0:
	d,p=heapq.heappop(Q)
	if p in c:
		continue
	c.add(p)
	for j,v in path[p]:
		if d+v<dist[j]:
			heapq.heappush(Q,(d+v,j))
			dist[j]=d+v
ans=0
for i in range(P):
	d,e=list(map(int,input().split()))
	d-=1
	ans=max(ans,(Y-dist[d])//e)
print(ans)