#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N = 1e6+6, inf = LLONG_MAX;
int n, m, sz, ans, dol[N], dol2[N], up[N], dep[N];
vector<int> path, adj[N];
bitset<N> vis;
set<pair<int, int>> st[N];
void dfs3(int x, int idx, int d) {
if (vis[x]) return;
vis[x] = 1;
auto it = st[idx].find(make_pair(-dep[x], x));
if (it != st[idx].end()) st[idx].erase(it);
for (int& y : adj[x]) {
if (y == up[x]) continue;
dfs3(y, idx, d+1);
}
}
void dfs2(int x) {
st[x].insert(make_pair(-dep[x], x));
for (int& y : adj[x]) {
if (y == up[x]) continue;
dfs2(y);
if (st[y].size() > st[x].size()) swap(st[x], st[y]);
for (pair<int, int> p : st[y]) {
st[x].insert(p);
}
st[y].clear();
}
//assert(dol2[x] <= st[x].size());
while (dol2[x] > 0) {
//cerr << x << " " << dol2[x] << endl;
dol2[x]--;
auto it = st[x].begin();
int d = it->first;
int y = it->second;
st[x].erase(it);
ans = min(ans, -d);
st[x].insert(make_pair(d+1, up[y]));
dfs3(y, x, 0);
}
}
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();
dol2[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));
memset(dol2, 0, sizeof(dol2));
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";
}
