//#pragma optimize("Ofast")
//#pragma optimize("unroll-loops")
//#pragma traget("avx,avx2")
#include <iostream>
#include <cmath>
#include <algorithm>
#include <stdio.h>
#include <cstdint>
#include <cstring>
#include <string>
#include <cstdlib>
#include <vector>
#include <bitset>
#include <map>
#include <queue>
#include <ctime>
#include <stack>
#include <set>
#include <list>
#include <random>
#include <deque>
#include <functional>
#include <iomanip>
#include <sstream>
#include <fstream>
#include <complex>
#include <numeric>
#include <cassert>
#include <array>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <thread>
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
#define el '\n'
#define ff first
#define ss second
#define pb push_back
#define pf push_front
#define popb pop_back
#define popf pop_front
#define point pair <ll, ll>
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
using namespace std;
#include <random>
mt19937 rnd(time(0));
const ll INF = 1e18 + 10;
const ll inf = 1e9 + 10;
const ll maxn = 2e5 + 10;
const ll mod = 1e9 + 7;
const ll K = 7e6 + 100;
const ll LOG = 20;
vector <ll> gr[maxn];
ll up[maxn][LOG], deep[maxn];
void dfs(ll v, ll pr) {
up[v][0] = pr;
for (ll i = 1; i < LOG; i++)
up[v][i] = up[ up[v][i - 1] ][i - 1];
for (ll to : gr[v]) {
if (to == pr)
continue;
deep[to] = deep[v] + 1;
dfs(to, v);
}
}
ll cnt[maxn], id[maxn], ans = inf;
multiset <ll> leaf[maxn];
void calc(ll v, ll pr) {
id[v] = v;
for (ll to : gr[v]) {
if (to == pr)
continue;
calc(to, v);
if (leaf[id[v]].size() < leaf[id[to]].size())
id[v] = id[to];
}
leaf[id[v]].insert(deep[v]);
for (ll to : gr[v]) {
if (to == pr || id[v] == id[to])
continue;
for (auto it : leaf[id[to]])
leaf[id[v]].insert(it);
}
while (cnt[v]) {
cnt[v]--;
auto it = leaf[id[v]].end();
it--;
ans = min(ans, *it);
leaf[id[v]].erase(it);
}
}
void solve() {
ll n, m;
cin >> n >> m;
for (ll i = 2; i <= n; i++) {
ll p;
cin >> p;
gr[i].pb(p);
gr[p].pb(i);
}
bool ok = 0;
dfs(1, 1);
for (ll i = 1; i <= m; i++) {
ll v, rem;
cin >> v;
rem = v;
if (v == 1)
ok = 1;
for (ll j = LOG - 1; j >= 0; j--)
if (deep[rem] < deep[up[v][j]] * 2)
v = up[v][j];
cnt[v]++;
}
if (ok)
return void(cout << 1 << el);
calc(1, 1);
cout << ans + 1 << el;
return;
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int tests = 1;
//cin >> tests;
while (tests--)
solve();
return 0;
}
/*
*/
