from heapq import heappop, heappush

def readints():
  return list(map(int, input().split()))

def deep_recursion():
  import sys
  import pypyjit  # type: ignore
  sys.setrecursionlimit(550000)
  pypyjit.set_param('max_unroll_recursion=-1')

def main():
  N, M, P, Y = readints()
  adj = [[] for _ in range(N)]
  for _ in range(M):
    a, b, c = readints()
    a -= 1
    b -= 1
    adj[a].append((b, c))
    adj[b].append((a, c))
  store = {}
  for _ in range(P):
    d, e = readints()
    store[d-1] = e

  INF = 10**18
  dist = [INF] * N
  dist[0] = 0
  q = [(0, 0)]
  ans = 0
  while q:
    d, u = heappop(q)
    if d > dist[u]:
      continue
    if u in store:
      ans = max(ans, max(Y - d, 0) // store[u])
    for v, c in adj[u]:
      if dist[u] + c < dist[v]:
        dist[v] = dist[u] + c
        heappush(q, (dist[v], v))
  print(ans)


if __name__ == '__main__':
  # deep_recursion()
  main()