#include <fstream>
#include <time.h>
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cstdlib>
#include <vector>
#include <bitset>
#include <queue>
#include <map>
#include <set>
#include <cmath>
#include <iomanip>
#define pb push_back
#define mp make_pair
#define sortvi(a) sort(a.begin(), a.end())
#define FOR(i, start, final) for (int i=start; i<=final; ++i)
#define ROF(i, start, final) for (int i=start; i>=final; --i)

#define f cin
#define g cout

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair < int, int> pi;
typedef vector< int > vi;
typedef vector< pair< int, int > > vpi;

int N, K, a[305], b[305], d[10005], sol, x, y;
vi G[10005];
bool srv[10005], inQ[10005];
queue < int > Q;

//ifstream f("test.in");ofstream g("test.out");

int main()
{
    f>>N>>K;
    FOR(i,1,N)d[i]=(1<<30);
    FOR(i,1,K)f>>a[i];
    FOR(i,1,K)f>>b[i], srv[b[i]]=1;
    FOR(i,2,N)
    {
        f>>x>>y;
        G[x].push_back(y);
        G[y].push_back(x);
    }

    FOR(i,1,K)
    {
        memset(inQ, 0, sizeof(inQ));
        Q.push(a[i]); inQ[a[i]]=1; d[a[i]]=0;
        while (Q.size())
        {
            int nod=Q.front(); Q.pop(); inQ[nod]=0;
            vector <int>::iterator it=G[nod].begin();
            for (; it!=G[nod].end(); ++it)
                if ( d[nod] + 1 < d[*it] )
                {
                    d[*it]=d[nod]+1;
                    //if (!inQ[*it])inQ[*it]=1,
                    Q.push(*it);
                }
        }
    }

    FOR(i,1,N)
        if (srv[i]==1)
            sol=max(sol, d[i]);

    g<<sol<<'\n';
    return 0;
}