from heapq import*
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-=1
    b-=1
    g[b].append((a,c))
    g[a].append((b,c))
q=[]
inf=1<<60
cost=[inf for _ in range(n)]
cost[0]=0
heappush(q,(0,0))
while q:
    c,now=heappop(q)
    if c>cost[now]:
        continue
    for nxt,v in g[now]:
        if cost[nxt]>cost[now]+v:
            cost[nxt]=cost[now]+v
            heappush(q,(cost[nxt],nxt))
ans=0
for i in range(p):
    d,e=map(int,input().split())
    d-=1
    ans=max((y-cost[d])//e,ans)
print(ans)