#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

//void f(int &x){char k;for(k=getchar();k<= 32;k=getchar());for(x=0;'0'<=k;k=getchar())x=x*10+k-'0';}


/*void f(int &x)//parsare cu semn
{
	char k; bool semn=0;
	for (k = getchar(); (k>'9' || k<'0');)
    {
        if (k=='-') semn=1;
        k=getchar();
    }
	for (x = 0; '0' <= k; k = getchar())	x = x * 10 + k - '0';
    if (semn==1) x=-x;
}*/


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[305], sol, x, y;
vector < int > G[1005];
bool srv[1005], inQ[1005];
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], Q.push(a[i]), d[a[i]]=0, inQ[a[i]]=1;
    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);
    }

    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,1,1005)
        if (srv[i]) sol=max(sol, d[i]);

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