N,M,P,Y=map(int,input().split())
edge=[[] for _ in range(N)]
for _ in range(M):
    a,b,c=map(int,input().split())
    a-=1
    b-=1
    edge[a].append((b,c))
    edge[b].append((a,c))

from heapq import *

D=[10**18]*N
D[0]=0
H=[(0,0)]
while H:
    cd,cp=heappop(H)
    if D[cp]<cd:
        continue
    for np,dd in edge[cp]:
        nd=cd+dd
        if D[np]>nd:
            D[np]=nd
            heappush(H,(nd,np))

ans=0
for _ in range(P):
    d,e=map(int,input().split())
    d-=1
    rest=Y-D[d]
    ans=max(ans,rest//e)

print(ans)