N,M,P,Y=map(int,input().split())
con=[[] for _ in range(N)]
for _ in range(M):
    A,B,C=map(int,input().split()) ; A-=1 ; B-=1
    con[A].append((B,C))
    con[B].append((A,C))

from heapq import heappush,heappop
def dijkstra(start):
    inf=1<<60 ; re=[inf for _ in range(len(con))] ; re[start]=0
    q=[] ; heappush(q,(0,start))
    while q:
        d,e=heappop(q)
        if re[e]<d : continue
        for aim,pd in con[e]:
            if d+pd<re[aim] : heappush(q,(d+pd,aim)) ; re[aim]=d+pd
    return re

dist=dijkstra(0)
ans=0
for _ in range(P):
    A,B=map(int,input().split()) ; A-=1
    ans=max(ans,(Y-dist[A])//B)
print(ans)