/*input
7 2
1
2
3
3
5
6
3 4
*/
#include <bits/stdc++.h>
#define pb push_back
#define x first
#define y second
#define all(a) a.begin(), a.end()
#define sz(a) (int)a.size()
#define rd() abs((int)rng())
using namespace std;
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
const int maxn = 2e5 + 100;
const int mod = 1e9 + 7;
mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
int n, m, has[maxn], dep[maxn], need[maxn], cnt[maxn], par[maxn];
vector<int>stk, ord;
vector<int>adj[maxn];
void prep(int v) {
stk.pb(v);
if (has[v]) {
++need[stk[dep[v] / 2 + 1]];
}
for (int to : adj[v]) {
prep(to);
}
stk.pop_back();
}
bool can(int min_dep) {
for (int v : ord) {
if (dep[v] >= min_dep) {
cnt[v]--;
}
if (cnt[v] > 0) {
return false;
}
cnt[par[v]] += cnt[v];
}
return true;
}
int main()
{
ios_base::sync_with_stdio(false), cin.tie(0);
cin >> n >> m;
for (int i = 2; i <= n; i++) {
int p;
cin >> p;
dep[i] = dep[p] + 1;
par[i] = p;
adj[p].pb(i);
}
for (int i = 1; i <= m; i++) {
int v;
cin >> v;
has[v] = 1;
}
for (int i = 1; i <= n; i++) {
ord.pb(i);
}
sort(all(ord), [&](int i, int j) {
return dep[i] > dep[j];
});
prep(1);
int lo = 0, hi = n - 1;
while (lo < hi) {
int mid = (lo + hi + 1) / 2;
for (int i = 1; i <= n; i++) {
cnt[i] = need[i];
}
if (can(mid)) {
lo = mid;
} else {
hi = mid - 1;
}
}
cout << lo + 1 << "\n";
return 0;
}
