#include <bits/stdc++.h>
using namespace std;
using lint = long long;
const int MAXN = 200005;
const int INF = 1e8;
int N, M;
int A[MAXN];
vector<int> adj[MAXN];
int depth[MAXN];
int lift[MAXN][20];
void DfsInit(int u) {
for (int j = 1; j < 20; j++) {
lift[u][j] = lift[lift[u][j - 1]][j - 1];
}
for (auto v : adj[u]) {
lift[v][0] = u;
depth[v] = depth[u] + 1;
DfsInit(v);
}
}
int dp[MAXN];
multiset<int> leaf[MAXN];
void Dfs(int u) {
dp[u] = INF;
leaf[u].emplace(depth[u]);
for (auto v : adj[u]) {
Dfs(v);
if (leaf[u].size() < leaf[v].size()) swap(leaf[u], leaf[v]);
for (auto i : leaf[v]) leaf[u].emplace(i);
leaf[v].clear();
dp[u] = min(dp[u], dp[v]);
}
while (A[u] > 0) {
A[u]--;
auto it = prev(end(leaf[u]));
dp[u] = min(dp[u], *it);
leaf[u].erase(it);
}
}
int main() {
ios::sync_with_stdio(0);
cin.tie(0), cout.tie(0);
cin >> N >> M;
for (int i = 2; i <= N; i++) {
int p;
cin >> p;
adj[p].emplace_back(i);
}
for (int i = 0; i < M; i++) {
int x;
cin >> x;
A[x]++;
}
if (A[1]) {
cout << 1 << "\n";
return 0;
}
DfsInit(1);
for (int i = 1; i <= N; i++) {
if (A[i]) {
int u = i;
for (int j = 19; j >= 0; j--) {
if (lift[u][j] != 0 && depth[lift[u][j]] > depth[i] - depth[lift[u][j]]) {
u = lift[u][j];
}
}
A[u]++;
A[i]--;
}
}
Dfs(1);
cout << 1 + dp[1] << "\n";
return 0;
}
