class UnionFind

  attr_accessor :nodes, :sizes

  def initialize(num)
    self.nodes = []
    self.sizes = []

    num.times do |n|
      self.nodes[n] = n
      self.sizes[n] = 1
    end
  end

  def root(i)
    # Loop up the chain until reaching root
    while nodes[i] != i do
      # path compression for future lookups
      nodes[i] = nodes[nodes[i]]
      i = nodes[i]
    end
    i
  end

  def union(i, j)
    rooti = root i
    rootj = root j

    # already connected
    return if rooti == rootj

    # root smaller to root of larger
    if sizes[i] < sizes[j]
      nodes[rooti] = rootj
      sizes[rootj] += sizes[rooti]
    else
      nodes[rootj] = rooti
      sizes[rooti] += sizes[rootj]
    end
  end

  def connected?(i, j)
    root(i) == root(j)
  end

end

N, M = gets.split.map &:to_i

uf = UnionFind.new(N)

pathes = N.times.map{ [] }
M.times{
  from, to = gets.split.map(&:to_i).map &:pred
  pathes[from] << to
}
S, T = gets.split.map(&:to_i).map &:pred

# p pathes

kpathes = N.times.map{ [] }
N.times{|i|
  f = -> from, n {
    if n < 1
      kpathes[i] << from
      # kpathes[from] << i
      return
    end
    pathes[from].each{|to|
      f[to, n - 1]
    }
  }
  f[i, 3]
}
kpathes.map! &:uniq
# p kpathes

N.times{|from|
  kpathes[from].each{|to|
    uf.union(from, to)
  }
}
if !uf.connected?(S, T)
  p -1
  exit
end

ans = 0
q = {}
q[S] = 1
used = [false] * N
used[S] = true
while !q.empty?
  ans += 1
  new_q = {}
  q.each_key{|from|
    kpathes[from].each{|to|
      next if used[to]
      if to == T
        p ans
        exit
      end
      new_q[to] = 1
    }
  }
  q = new_q
end
p -1