#include <bits/stdc++.h>
using namespace std;
int n, L;
long long S;
const int N = 1e5 + 10;
vector<int> G[N];
long long w[N];
int d[N], ud[N];
long long dat[N * 4];
void update(int x, long long y, int v = 0, int l = 0, int r = n) {
  dat[v] += y;
  if (l + 1 == r) return;
  int mid = (l + r) / 2, chl = v * 2 + 1, chr = v * 2 + 2;
  if (x < mid)
    update(x, y, chl, l, mid);
  else
    update(x, y, chr, mid, r);
}
int query(long long S, int v = 0, int l = 0, int r = n) {
  if (l + 1 == r) return l;
  int mid = (l + r) / 2, chl = v * 2 + 1, chr = v * 2 + 2;
  if (dat[chl] >= S) return query(S, chl, l, mid);
  return query(S - dat[chl], chr, mid, r);
}
void Maxi(int &a, int b) {
  if (b > a) a = b;
}
void dfs(int v = 1, int depth = 0) {
  d[v] = depth;
  update(depth, w[v]);
  long long tot = dat[0];
  if (tot <= S) {
    ud[v] = 0;
  } else {
    ud[v] = query(tot - S) + 1;
  }
  Maxi(ud[v], depth - L + 1);
  for (auto u : G[v]) {
    dfs(u, depth + 1);
  }
  update(depth, -w[v]);
}
int ans = 0;
void Mini(int &a, int b) {
  if (b < a) a = b;
}
int solve(int v = 1) {
  int up = d[v] + 1;
  for (auto u : G[v]) {
    Mini(up, solve(u));
  }
  if (up <= d[v]) return up;
  ans++;
  return ud[v];
}
int main() {
  scanf("%d%d%lld", &n, &L, &S);
  for (int i = 1; i <= n; i++) {
    scanf("%lld", w + i);
    if (w[i] > S) {
      return puts("-1"), 0;
    }
  }
  for (int i = 2; i <= n; i++) {
    int p;
    scanf("%d", &p);
    G[p].push_back(i);
  }
  dfs();
  solve();
  printf("%d\n", ans);
}
