#include<cstdio>
#include<queue>
#include<cstring>
#include<algorithm>
using namespace std;
typedef long long ll;
queue<int>q;
int v[1000010],first[100010],next[1000010],en;
ll ans=2147483647ll;
void AddEdge(int U,int V){v[++en]=V;next[en]=first[U];first[U]=en;
}
int n,m,K,a[100010],b[100010],dis1[100010],dis2[100010],dis3[100010],eu[1000010],ev[1000010],es;
bool inq[100010];
void spfa1(int dis[])
{memset(dis,0x7f,sizeof(int)*100010);memset(inq,0,sizeof(inq));for(int i=1;i<=m;++i){dis[a[i]]=0;q.push(a[i]);inq[a[i]]=1;}while(!q.empty()){int U=q.front();for(int i=first[U];i;i=next[i])if(dis[v[i]]>dis[U]+1){dis[v[i]]=dis[U]+1;if(!inq[v[i]]){q.push(v[i]);inq[v[i]]=1;}}q.pop(); inq[U]=0;}
}
void spfa2(int dis[])
{memset(dis,0x7f,sizeof(int)*100010);memset(inq,0,sizeof(inq));for(int i=1;i<=K;++i){dis[b[i]]=0;q.push(b[i]);inq[b[i]]=1;}while(!q.empty()){int U=q.front();for(int i=first[U];i;i=next[i])if(dis[v[i]]>dis[U]+1){dis[v[i]]=dis[U]+1;if(!inq[v[i]]){q.push(v[i]);inq[v[i]]=1;}}q.pop(); inq[U]=0;}
}
void spfa3(int dis[]){memset(dis,0x7f,sizeof(int)*100010);memset(inq,0,sizeof(inq));dis[1]=0; q.push(1); inq[1]=1;while(!q.empty()){int U=q.front();for(int i=first[U];i;i=next[i])if(dis[v[i]]>dis[U]+1){dis[v[i]]=dis[U]+1;if(!inq[v[i]]){q.push(v[i]);inq[v[i]]=1;}}q.pop(); inq[U]=0;}
}
int main(){
//  freopen("i.in","r",stdin);scanf("%d%d%d",&n,&m,&K);for(int i=1;i<=m;++i){scanf("%d",&a[i]);}for(int i=1;i<=K;++i){scanf("%d",&b[i]);}int x,y;for(int i=1;i<=n;++i){scanf("%d",&x);for(int j=1;j<=x;++j){scanf("%d",&y);AddEdge(y,i);eu[++es]=i;ev[es]=y;}}spfa1(dis1);spfa2(dis2);memset(first,0,sizeof(first));memset(next,0,sizeof(next));memset(v,0,sizeof(v));en=0;for(int i=1;i<=es;++i){AddEdge(eu[i],ev[i]);}spfa3(dis3);for(int i=1;i<=n;++i){ans=min(ans,(ll)dis3[i]+(ll)dis1[i]+(ll)dis2[i]);}if(ans<=2000000000ll){printf("%d\n",(int)ans);}else{puts("impossible");}return 0;
}