#include <bits/stdc++.h>
using namespace std;
const int N = 2e5+5, inf = INT_MAX;
int n, m, sz, ans, dol[N], up[N], dep[N];
vector<int> path, adj[N];
bitset<N> vis;
void dfs3(int x, set<pair<int, int>>& st, int d) {
if (d) {
if (vis[x]) return;
vis[x] = 1;
auto it = st.find(make_pair(-dep[x], x));
if (it != st.end()) st.erase(it);
}
for (int& y : adj[x]) {
if (y == up[x]) continue;
dfs3(y, st, d+1);
}
}
set<pair<int, int>> dfs2(int x) {
set<pair<int, int>> st;
st.insert(make_pair(-dep[x], x));
for (int& y : adj[x]) {
if (y == up[x]) continue;
set<pair<int, int>> st2 = dfs2(y);
if (st2.size() > st.size()) swap(st, st2);
for (pair<int, int> p : st2) {
st.insert(p);
}
st2.clear();
}
while (dol[x] > 0) {
dol[x]--;
auto it = st.begin();
int d = it->first;
int y = it->second;
st.erase(it);
ans = min(ans, -d);
dfs3(up[y], st, 0);
st.insert(make_pair(d+1, up[y]));
}
return st;
}
void dfs(int x, int u, int d) {
up[x] = u;
dep[x] = d;
path.push_back(x);
if (dol[x] && x) {
dol[x]--;
int sz = path.size();
dol[path[sz - sz/2]]++;
}
for (int& y : adj[x]) {
if (y == u) continue;
dfs(y, x, d+1);
}
path.pop_back();
}
signed main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
sz = 0;
ans = inf;
memset(dol, 0, sizeof(dol));
cin >> n >> m;
for (int i = 1; i < n; i++) {
int x;
cin >> x;
x--;
adj[x].push_back(i);
adj[i].push_back(x);
}
bool ok = 1;
for (int i = 0; i < m; i++) {
int x;
cin >> x;
x--;
dol[x]++;
if (!x) ok = 0;
}
if (n == 1 || !ok) {
cout << "1\n";
return 0;
}
dfs(0, 0, 1);
dfs2(0);
cout << ans << "\n";
}
