#include <atcoder/all>
#include <bits/stdc++.h>
#define rep(i, a, b) for (ll i = (ll)(a); i < (ll)(b); i++)
using namespace atcoder;
using namespace std;

typedef long long ll;

int main() {
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(0);
    int n, t;
    cin >> n >> t;
    int power = 0;
    int nw = t;
    vector<int> ts(n - 1), p(n - 1);
    rep(i, 0, n - 1) cin >> ts[i];
    int m;
    cin >> m;
    while (m--) {
        int x;
        cin >> x;
        p[x - 2] = 1;
    }
    int ans = 0;
    rep(i, 0, n - 1) {
        t -= ts[i];
        while (t <= 0 && power) {
            t += 10;
            ans++;
            power--;
        }
        if (t <= 0) {
            cout << -1 << endl;
            return 0;
        }
        power += p[i];
    }
    cout << ans << endl;
}