#include<cstdio>

 #include<iostream>

 #include<cstring>

 #include<algorithm>

 #include<queue>

 #define ll long long

 #define ui unsigned int

 using namespace std;

 const int maxn=;

 struct zs{int too,pre;}e[maxn<<];int tot,last[maxn];

 int zt[maxn],deg[maxn];

 int i,j,k,n,m,ans;

 bool gg[maxn],GG;

 int ra,fh;char rx;

 inline int read(){

     rx=getchar(),ra=,fh=;

     while((rx<''||rx>'')&&rx!='-')rx=getchar();

     if(rx=='-')fh=-,rx=getchar();

     while(rx>=''&&rx<='')ra*=,ra+=rx-,rx=getchar();return ra*fh;

 }

 inline void insert(int a,int b){

     deg[a]++,deg[b]++;

     e[++tot].too=b,e[tot].pre=last[a],last[a]=tot,

     e[++tot].too=a,e[tot].pre=last[b],last[b]=tot;

 }

 void dfs(int x,int fa){

     zt[x]=;

     int num[]={,,};

     for(int i=last[x];i;i=e[i].pre)if(e[i].too!=fa)

         dfs(e[i].too,x),num[zt[e[i].too]]++;

     if(gg[x]&&deg[x]==)GG=;

     if(gg[x])zt[x]=,ans+=num[];

     else if(num[]&&num[])zt[x]=,ans++;

     else if(num[]||num[])zt[x]=num[]?:;

     else if(num[])zt[x]=;

 //    printf("x:%d   zt:%d\n",x,zt[x]);

 }

 int main(){

     n=read()+,m=read();

     for(i=;i<n;i++)insert(read()+,read()+);

     for(i=;i<=m;i++)gg[read()+]=;

     for(i=n;i;i--)if(deg[i]==){dfs(i,),ans+=zt[i]==;break;}

     printf("%d\n",!GG?ans:-);

 }