from heapq import*
N,M,P,Y=map(int,input().split())
gr=[[] for i in range(N)]
for i in range(M):
    A,B,C=map(int,input().split())
    gr[A-1].append((B-1,C))
    gr[B-1].append((A-1,C))
dist=[1<<60]*N
dist[0]=0
hq=[(0,0)]
while hq:
    dis,pos=heappop(hq)
    if dist[pos]!=dis:
        continue
    for t,c in gr[pos]:
        if dist[t]>dis+c:
            dist[t]=dis+c
            heappush(hq,(dist[t],t))
ans=0
for i in range(P):
    D,E=map(int,input().split())
    D-=1
    now=max(0,Y-dist[D])
    ans=max(ans,now//E)
print(ans)