#include <queue>
#include <vector>
#include <iostream>
#include <functional>
using namespace std;
int solve(int N, int K, vector<int> S, vector<int> V) {
int bits = 1;
while ((1 << bits) <= N) ++bits;
vector<vector<int> > P(bits, vector<int>(N));
vector<int> depth(N);
vector<vector<int> > G(N);
for (int i = 1; i < N; ++i) {
P[0][i] = S[i - 1];
depth[i] = depth[P[0][i]] + 1;
G[P[0][i]].push_back(i);
}
vector<bool> coin(N);
for (int i = 0; i < K; ++i) {
coin[V[i]] = true;
}
for (int i = 1; i < bits; ++i) {
for (int j = 0; j < N; ++j) {
P[i][j] = P[i - 1][P[i - 1][j]];
}
}
int cnt = 0;
vector<int> sl(N), sr(N), ord;
function<void(int)> eulertour = [&](int pos) {
ord.push_back(pos);
sl[pos] = cnt++;
for (int i : G[pos]) {
eulertour(i);
}
sr[pos] = cnt;
};
function<int(int, int)> ancestor = [&](int pos, int level) {
for (int i = bits - 1; i >= 0; --i) {
if (level >= (1 << i)) {
pos = P[i][pos];
level -= (1 << i);
}
}
return pos;
};
eulertour(0);
if (coin[0]) return 1;
int L = 0, R = N + 1;
vector<vector<int> > query(N);
for (int i = 1; i < N; ++i) {
if (!coin[i]) continue;
int pos = ancestor(i, (depth[i] - 1) / 2);
query[sl[pos]].push_back(sr[pos]);
}
while (R - L > 1) {
int M = (L + R) >> 1;
priority_queue<int, vector<int>, greater<int> > que;
bool ok = true;
for (int i = 0; i < N && ok; ++i) {
for (int j : query[i]) {
que.push(j);
}
if (depth[ord[i]] >= M && !que.empty()) {
int u = que.top(); que.pop();
if (u <= i) ok = false;
}
}
if (!que.empty()) ok = false;
if (ok) L = M;
else R = M;
}
return R;
}
int main() {
cin.tie(0);
ios_base::sync_with_stdio(false);
int N, K;
cin >> N >> K;
vector<int> S(N - 1);
for (int i = 0; i < N - 1; ++i) {
cin >> S[i]; --S[i];
}
vector<int> V(K);
for (int i = 0; i < K; ++i) {
cin >> V[i]; --V[i];
}
int ans = solve(N, K, S, V);
cout << ans << '\n';
return 0;
}
