#include <iostream>
#include <vector>
using namespace std;

int main() {
    int N, T;
    cin >> N >> T;
    vector<int> t(N - 1);
    for (int i = 0; i < N - 1; i++) {
        cin >> t[i];
    }
    int K;
    cin >> K;
    vector<int> x(K);
    for (int i = 0; i < K; i++) {
        cin >> x[i];
        x[i]--;
    }
    vector<bool> has_clock(N, false);
    for (int i = 0; i < K; i++) {
        has_clock[x[i]] = true;
    }
    int count = T;
    for (int i = 1; i < N; i++) {
        count -= t[i - 1];
        if (count <= 0) {
            cout << -1 << endl;
            return 0;
        }
        if (has_clock[i]) {
            count += 10;
        }
    }
    cout << max(0, K - (count - 1) / 10) << endl;
}