/*
 ID: bradyawn
 PROG: contest
 LANG: C++11
 */

#include <iostream>
#include <algorithm>
#include <iomanip>
#include <fstream>
#include <vector>
#include <deque>
#include <string>
#include <cmath>
#include <map>
#include <unordered_map>
#include <utility>
#include <set>
#include <unordered_set>
#include <ctime>
#include <queue>
#include <stack>
#include <bitset>
#include <random>
#include <cstring>
#include <complex>
#include <cassert>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<int,int> i2;
typedef pair<ll,ll> ll2;

ll n, L, S;

int w[100001];
int p[100001];
int d[100001];
bool vis[100001];
vector<int> at[100001];

int ret = 0;

void solve(int cur)
{
    if (vis[cur]) return;
    ret++;
    ll weight = S;
    int cnt = L;
    while (weight-w[cur] >= 0 && cnt-1 >= 0 && cur)
    {
        weight -= w[cur];
        cnt--;
        vis[cur] = 1;
        cur = p[cur];
    }
//    solve(cur);
}

int main()
{
    //ifstream inf("");
    //ofstream outf("");
    cout << setprecision(10);
    ios::sync_with_stdio(0); cin.tie(0);

    cin >> n >> L >> S; //mxlen, mxsum

    for (int i = 1; i <= n; i++)
    {
        cin >> w[i];
        if (w[i] > S)
        {
            cout << -1 << '\n';
            return 0;
        }
    }
    for (int i = 2; i <= n; i++)
    {
        cin >> p[i];
        d[i] = d[p[i]]+1;
        at[d[i]].push_back(i);
    } at[0].push_back(1);

    for (int dep = n; dep >= 0; dep--)
        for (auto e : at[dep]) solve(e);

    cout << ret << '\n';

    return 0;
}