#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
const int mod2 = 1e9 + 9;
inline int pow_(long long a, int n, int p = mod) {
  int r = 1;
  while (n) {
    if (n & 1) r = r * a % p;
    n >>= 1;
    a = a * a % p;
  }
  return r;
}
int main(void) {
  int n, l;
  long long s;
  cin >> n >> l >> s;
  int w[n + 1], p[n + 1];
  memset(p, 0, sizeof(p));
  memset(w, 0, sizeof(w));
  for (int i = 1; i <= n; i++) cin >> w[i];
  for (int i = 2; i <= n; i++) cin >> p[i];
  bool done[n + 1];
  memset(done, false, sizeof(done));
  for (int i = 1; i <= n; i++)
    if (w[i] > s) return 0 * printf("-1\n");
  int ans = 0;
  for (int i = n; i >= 1; i--) {
    if (done[i]) continue;
    ans++;
    int cur = i;
    int L = l;
    long long S = s;
    while (cur and L > 0 and S >= w[cur]) {
      done[cur] = true;
      S -= w[cur];
      L--;
      cur = p[cur];
    }
  }
  cout << ans << "\n";
}
