n64, m, p64, y = read_line.split.map(&.to_i64)
n = n64.to_i
p = p64.to_i
g = Array.new(n) { [] of Tuple(Int32, Int64) }
m.to_i.times do
  a, b, c = read_line.split.map(&.to_i)
  g[a - 1] << {b - 1, c.to_i64}
  g[b - 1] << {a - 1, c.to_i64}
end
price = Array.new(n, 2e18.to_i64)
p.times do
  d, e = read_line.split.map(&.to_i)
  price[d - 1] = e
end
dist = Array.new(n, 1e18.to_i64)
dist[0] = 0
ans = 0i64
q = PriorityQueue(Tuple(Int64, Int32)).new(n)
q.add({0i64, 0})
while q.size > 0
  d, cur = q.pop
  d *= -1
  next if d > dist[cur]
  ans = {ans, (y - dist[cur]) // price[cur]}.max
  g[cur].each do |adj, cost|
    nd = d + cost
    next if dist[adj] <= nd
    dist[adj] = nd
    q.add({-nd, adj})
  end
end
puts ans

class PriorityQueue(T)
  def initialize(capacity : Int32)
    @elem = Array(T).new(capacity)
  end

  def initialize(list : Enumerable(T))
    @elem = list.to_a
    1.upto(size - 1) { |i| fixup(i) }
  end

  def size
    @elem.size
  end

  def add(v)
    @elem << v
    fixup(size - 1)
  end

  def top
    @elem[0]
  end

  def pop
    ret = @elem[0]
    last = @elem.pop
    if size > 0
      @elem[0] = last
      fixdown(0)
    end
    ret
  end

  def clear
    @elem.clear
  end

  def decrease_top(new_value : T)
    @elem[0] = new_value
    fixdown(0)
  end

  def to_s(io : IO)
    io << @elem
  end

  private def fixup(index : Int32)
    while index > 0
      parent = (index - 1) // 2
      break if @elem[parent] >= @elem[index]
      @elem[parent], @elem[index] = @elem[index], @elem[parent]
      index = parent
    end
  end

  private def fixdown(index : Int32)
    while true
      left = index * 2 + 1
      break if left >= size
      right = index * 2 + 2
      child = right >= size || @elem[left] > @elem[right] ? left : right
      if @elem[child] > @elem[index]
        @elem[child], @elem[index] = @elem[index], @elem[child]
        index = child
      else
        break
      end
    end
  end
end