from __future__ import print_function
import heapq, sys

def dijkstra(G, s):
  V, E = G; d = {}; S = set(); q = []
  for u in V: d.setdefault(u, None)
  for u in s:
    d[u] = 0; heapq.heappush(q, (0,u))
  while q:
    n, u = heapq.heappop(q)
    if u in S: continue
    S.add(u)
    for v in E[u]:
      if d[v] is None or d[v] > d[u] + 1:
        d[v] = d[u] + 1; heapq.heappush(q, (d[v],v))
  return d

def transpose(G):
  V, E = G; ET = {}
  for u in V: ET[u] = []
  for u, vs in E.iteritems():
    for v in vs: ET[v].append(u)
  return V, ET

n, m, k = map(int, sys.stdin.readline().split())
iron    = [ int(x)-1 for x in sys.stdin.readline().split() ]
coal    = [ int(x)-1 for x in sys.stdin.readline().split() ]
V, E    = range(n), {}
for i, line in enumerate(sys.stdin):
  E[i] = [ int(x)-1 for x in line.split()[1:] ]
G       = V, E
GT      = transpose(G)
ds      = dijkstra(G, [0])
di      = dijkstra(GT, iron)
dc      = dijkstra(GT, coal)
try:
  m = min( ds[i]+di[i]+dc[i] for i in xrange(n)
           if None not in (ds[i],di[i],dc[i]) )
  print(m)
except ValueError as e:
  if "empty" not in e.args[0]: raise
  print("impossible")