#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 2e5+5;
int n, m;
vector<int> adj[N];
bool coin[N];
void read() {
cin >> n >> m;
for(int i = 2; i <= n; i++) {
int x; cin >> x;
adj[x].push_back(i);
adj[i].push_back(x);
}
memset(coin, 0, sizeof coin);
for(int i = 0; i < m; i++) {
int x; cin >> x;
coin[x] = 1;
}
}
const int lv = 20;
int par[N][lv+1];
int fi[N], se[N], depth[N];
vector<int> euler;
int timer;
void dfs(int u, int p, int h) {
euler.push_back(u);
fi[u] = timer++;
depth[u] = h;
par[u][0] = p;
for(int i = 1; i <= lv; i++)
par[u][i] = par[par[u][i-1]][i-1];
for(int v : adj[u]) if(v != p)
dfs(v, u, h+1);
se[u] = timer;
}
int ancestor(int u, int x) {
for(int i = lv; i >= 0; i--) {
if(x - (1<<i) >= 0)
u = par[u][i], x -= (1<<i);
}
return u;
}
void solve() {
if(coin[1]) {
cout << 1 << '\n';
return;
}
timer = 0;
dfs(1, 1, 0);
vector<vector<int>> queries(n+1);
for(int i = 1; i <= n; i++) {
if(!coin[i]) continue;
int anc = ancestor(i, (depth[i] - 1) / 2);
queries[fi[anc]].push_back(se[anc]);
}
int ans = 0;
int l = 0, r = n;
while(l <= r) {
int mid = (l+r)/2;
bool ok = 1;
priority_queue<int, vector<int>, greater<int>> pq;
for(int i = 0; i < n && ok; i++) {
for(int j : queries[i])
pq.push(j);
if(depth[euler[i]] >= mid && !pq.empty()) {
int x = pq.top(); pq.pop();
if(x <= i) ok = 0;
}
}
if(!pq.empty()) ok = 0;
if(ok) {
ans = mid, l = mid + 1;
} else {
r = mid - 1;
}
}
cout << ans + 1 << '\n';
}
int main() {
ios::sync_with_stdio(0); cin.tie(0);
read();
solve();
return 0;
}
