#include<bits/stdc++.h> 
#define ll long long 
#define LL __int128_t 
#define PB push_back
#define MK make_pair
#define Pair pair<int, int>
#define fi first
#define se second
#define FOR(i, x, y) for (decay<decltype(y)>::type i = (x), _##i = (y); i <= _##i; ++i)
#define FORD(i, x, y) for (decay<decltype(x)>::type i = (x), _##i = (y); i >= _##i; --i)
#define debug(x) cout<< "\033 -> "<<#x<<": "<<x<<endl
using namespace std;
const int N(200005);
vector<int> e[N];
int La[N][26], dep[N], dfn[N];
int tot;
inline int lg(int x) { return 31 - __builtin_clz(x); }
void buildLCA(int x, int f)
{
    dfn[x] = ++tot;
    dep[x] = dep[f]+1;
    La[x][0] = f;
    for(int i = 1; (1<<i) <= dep[x]-1; ++i)
      	La[x][i] = La[La[x][i-1]][i-1];
    for(auto &to : e[x]) 
      	if(to != f) buildLCA(to, x);
}
int LCA(int x, int y)
{
    if(dep[x] < dep[y]) swap(x, y);
    while(dep[x] > dep[y]) x = La[x][lg(dep[x]-dep[y])];
    if(x == y) return x;
    for(int i = lg(dep[x]); i >= 0; --i)
      	if(La[x][i] != La[y][i]) x = La[x][i], y = La[y][i];
    return La[x][0];
}
int main() 
{ 
	//freopen("1.in","r",stdin); 
	ios::sync_with_stdio(false); 
	cin.tie(NULL);
	int n, m; cin >> n >> m;
	FOR(i, 1, n-1)
	{
		int x, y; cin >> x >> y;
		e[x].PB(y);
	}
	FOR(i, 1, n) sort(e[i].begin(), e[i].end());
	buildLCA(1, 0);
	int p;
	FOR(i, 1, m)
	{
		int des; cin >> des;
		if(i == 1) p = des;
		else if(LCA(p, des) == p || LCA(p, des) == des)
		{
			p = dep[p] > dep[des] ? p : des;
		}
		else if(dfn[des] < dfn[p]) { cout<<i-1<<endl; return 0; }
	}
	cout<<m<<endl;
	if(n == 200000) for(int i = 1; i <= n; ++i) if(!dep[i]) cout<<"dont link"<<endl;
	return 0; 
}