#include <bits/stdc++.h>
using namespace std;
int N, M;
vector<int> adj[200005];
priority_queue<int, vector<int>> pq[200005];
int need[200005], can[200005], depth[200005];
bitset<200005> coin;
void DFS(int u, int p) {
depth[u] = depth[p] + 1;
for(int v : adj[u]) {
if(v == p) continue;
DFS(v, u);
if(pq[v].size() > pq[u].size()) swap(pq[u], pq[v]);
need[u] += need[v];
}
for(int v : adj[u]) {
if(v == p) continue;
while(!pq[v].empty()) pq[u].emplace(pq[v].top()), pq[v].pop();
}
if(coin[u]) pq[u].emplace(depth[u] / 2 + (depth[u] & 1));
while(!pq[u].empty() && pq[u].top() == depth[u]) {
need[u]++;
pq[u].pop();
}
// cout << u << " " << need[u] << "\n";
}
int l, r, mid, ans;
bool ok;
void check(int u, int p) {
if(depth[u] >= mid) can[u] = 1;
else can[u] = 0;
for(int v : adj[u]) {
if(v == p) continue;
check(v, u);
can[u] += can[v];
}
if(can[u] < need[u]) ok = 0;
}
int main() {
depth[0] = -2;
ios_base::sync_with_stdio(0);
cin >> N >> M;
int u;
for(int i = 2; i <= N; i++) {
cin >> u;
adj[u].push_back(i);
adj[i].push_back(u);
}
for(int i = 1; i <= M; i++) {
cin >> u;
coin[u] = 1;
}
if(coin[1] == 1) {
cout << "1\n";
exit(0);
}
DFS(1, 0);
l = -1, r = N - 2;
while(l <= r) {
mid = (l + r) >> 1;
ok = 1;
check(1, 0);
if(ok) {
ans = mid;
l = mid + 1;
}else r = mid - 1;
}
cout << ans + 2 << "\n";
cin >> N;
}
