#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 10;
const int logn = 19;
vector <int> g[maxn];
int par[maxn];
int dep[maxn];
int cnt[maxn];
int anc[logn + 1][maxn];
int lift(int x, int depth) {
for(int i = logn; i >= 0; i--) {
if(dep[x] - (1 << i) > depth) {
x = anc[i][x];
}
}
return x;
}
void dfs(int x) {
for(int i : g[x]) {
dep[i] = 1 + dep[x];
dfs(i);
}
}
int ans = INT_MAX;
void solve(int x, priority_queue <int> &s) {
s.push(dep[x]);
for(int i : g[x]) {
priority_queue <int> t;
solve(i, t);
if(t.size() > s.size()) t.swap(s);
while(not t.empty()) {
s.push(t.top());
t.pop();
}
}
for(int j = 0; j < cnt[x]; j++) {
ans = min(ans, s.top());
s.pop();
}
}
int main() {
int n, m;
cin >> n >> m;
for(int i = 2; i <= n; i++) {
cin >> par[i];
g[par[i]].push_back(i);
anc[0][i] = par[i];
}
for(int i = 1; i <= logn; i++) {
for(int j = 1; j <= n; j++) {
anc[i][j] = anc[i - 1][anc[i - 1][j]];
}
}
dep[1] = 1;
dfs(1);
vector <int> v;
for(int i = 1; i <= m; i++) {
int x;
cin >> x;
v.emplace_back(x);
if(x == 1) {
cout << 1 << endl;
exit(0);
}
}
for(auto i : v) {
int top = (dep[i] + 1) / 2;
i = lift(i, top);
cnt[i] += 1;
}
priority_queue <int> s;
solve(1, s);
cout << ans << endl;
return 0;
}
