#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e5 + 10;
const int LOGN = 19;
vector<int> v[MAXN];
int n, m, tick;
int dsc[MAXN], fin[MAXN], lvl[MAXN], dad[MAXN][LOGN];
inline pair<int, int> maxp(const pair<int, int> &a, const pair<int, int> &b) {
if (a > b) return a;
return b;
}
struct Tournament {
int offset;
vector<pair<int, int>> t;
Tournament(int sz) {
for (offset = 1; offset <= sz; offset *= 2);
t.resize(2 * offset);
}
void ins(int i, int val) {
i += offset; t[i] = {val, i - offset};
for (i /= 2; i > 0; i /= 2)
t[i] = maxp(t[2 * i], t[2 * i + 1]);
}
pair<int, int> max(int node, int from, int to, int lo, int hi) {
if (to <= lo || hi <= from) return {-1, -1};
if (from >= lo && to <= hi) return t[node];
return maxp(max(2 * node, from, (from + to) / 2, lo, hi),
max(2 * node + 1, (from + to) / 2, to, lo, hi));
}
pair<int, int> max(int lo, int hi) {
return max(1, 0, offset, lo, hi);
}
};
inline void init() {
for (int j = 1; j < LOGN; ++j)
for (int i = 0; i < n; ++i)
dad[i][j] = dad[dad[i][j - 1]][j - 1];
}
void dfs(int node, int l) {
++tick;
dsc[node] = tick;
lvl[node] = l;
for (int nxt : v[node]) {
if (nxt == dad[node][0]) continue;
dfs(nxt, l + 1);
}
fin[node] = tick;
}
int up(int node, int k) {
for (int j = LOGN - 1; j >= 0; --j) {
if ((1 << j) <= k) {
node = dad[node][j];
k -= 1 << j;
}
}
return node;
}
int main(void) {
scanf("%d%d", &n, &m);
for (int i = 1; i < n; ++i) {
scanf("%d", &dad[i][0]);
--dad[i][0];
v[dad[i][0]].push_back(i);
}
init();
dfs(0, 0);
vector<pair<int, int>> I;
for (int i = 0; i < m; ++i) {
int x;
scanf("%d", &x); --x;
x = up(x, (lvl[x] - 1) / 2);
I.emplace_back(dsc[x], fin[x]);
}
sort(I.begin(), I.end(),
[&](const pair<int, int> &A, const pair<int, int> &B) {
return A.second - A.first < B.second - B.first;
});
Tournament T(n + 10);
for (int i = 0; i < n; ++i)
T.ins(dsc[i], lvl[i]);
int sol = n + 10;
for (const auto &p : I) {
auto mx = T.max(p.first, p.second + 1);
sol = min(sol, mx.first);
T.ins(mx.second, 0);
}
printf("%d\n", sol + 1);
return 0;
}
