#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
long long n, l, s;
long long w[100000 + 10];
long long p[100000 + 10];
vector<long long> child[100000 + 10];
long long dp1[100000 + 10];
set<pair<long long, long long> > dp2[100000 + 10];
void dfs(long long x) {
  long long sum = 0;
  set<pair<long long, long long> > temp;
  for (auto it = child[x].begin(); it != child[x].end(); it++) {
    dfs((*it));
    sum += dp1[(*it)];
    for (auto it1 = dp2[(*it)].begin(); it1 != dp2[(*it)].end(); it1++) {
      temp.insert((*it1));
    }
  }
  bool valid = 0;
  for (auto it = temp.begin(); it != temp.end(); it++) {
    if ((*it).first + w[x] <= s and (*it).second < l) {
      valid = 1;
      break;
    }
  }
  if (!valid) {
    dp2[x].insert(make_pair(w[x], 1));
    dp1[x] = sum + 1;
    return;
  }
  dp1[x] = sum;
  long long len = n + 1;
  for (auto it = temp.begin(); it != temp.end(); it++) {
    if ((*it).second < min(len, l) and (*it).first + w[x] <= s) {
      len = (*it).second;
      dp2[x].insert(make_pair((*it).first + w[x], (*it).second + 1));
    }
  }
}
int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cin >> n >> l >> s;
  bool check = 0;
  for (long long i = 1; i < n + 1; i++) {
    cin >> w[i];
    if (w[i] > s) {
      check = 1;
    }
  }
  for (long long i = 2; i < n + 1; i++) {
    cin >> p[i];
    child[p[i]].push_back(i);
  }
  if (check) {
    cout << -1 << '\n';
    return 0;
  }
  dfs(1);
  cout << dp1[1] << '\n';
  return 0;
}
