#include <vector>
#include <iostream>
#include <tuple>
#include <algorithm>
#include <cassert>

using namespace std;

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

template<typename T>
struct RMQ {
	vector<vector<T>> jmp;
	RMQ(const vector<T>& V) : jmp(1, V) {
		for (int pw = 1, k = 1; pw * 2 <= sz(V); pw *= 2, ++k) {
			jmp.emplace_back(sz(V) - pw * 2 + 1);
			rep(j,0,sz(jmp[k]))
				jmp[k][j] = min(jmp[k - 1][j], jmp[k - 1][j + pw]);
		}
	}
	T query(int a, int b) {
		assert(a < b); // or return inf if a == b
		int dep = 31 - __builtin_clz(b - a);
		return min(jmp[dep][a], jmp[dep][b - (1 << dep)]);
	}
};


struct LCA {
	int T = 0;
	vi time, path, ret;
	RMQ<int> rmq;

	LCA(vector<vi>& C) : time(sz(C)), rmq((dfs(C,0,-1), ret)) {}
	void dfs(vector<vi>& C, int v, int par) {
		time[v] = T++;
		for (int y : C[v]) if (y != par) {
			path.push_back(v), ret.push_back(time[v]);
			dfs(C, y, v);
		}
	}

	int lca(int a, int b) {
		if (a == b) return a;
		tie(a, b) = minmax(time[a], time[b]);
		return path[rmq.query(a, b)];
	}
};

int main()
{
    int n, m;
    std::cin >> n >> m;
    std::vector<int> parent(n);
    parent[0] = -1;
    for(int i=0; i<n-1; i++)
    {
        int a, b;
        std::cin >> a >> b;
        parent[b-1] = a-1;
    }
    
    std::vector<std::vector<int> > children(n);
    
    for(int i=1; i<n; i++)
    {
        children[parent[i]].push_back(i);
    }
    
    LCA lca(children);
    
    std::vector<int> availableroad(n);
    
    int cur = 0;
    for(int i=0; i<m; i++)
    {
        int d;
        std::cin >> d;
        int a = lca.lca(d-1, cur);
        if(a != cur)
        {
            int nc = children[a].size();
            int j=0;
            for(j=availableroad[a]; j<nc; j++)
            {
                if(lca.lca(cur, children[a][j]) == children[a][j])
                {
                    break;
                }
            }
            availableroad[a] = j;            
        }
        if(a != d-1)
        {
            int nc = children[a].size();
            bool ok = false;
            int j=0;
            for(j=availableroad[a]; j<nc; j++)
            {
                if(lca.lca(d-1, children[a][j]) == children[a][j])
                {
                    ok = true;
                    break;
                }
            }
            if(!ok)
            {
                std::cout << i << std::endl;
                return 0;
            }
            availableroad[a] = j;            
            cur = d-1;
        }
    }
    std::cout << m << std::endl;
}