#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
	std::ios_base::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int N, T;
    cin >> N >> T;
    vector<int> t(N - 1);
    for(auto& e: t) cin >> e;
    int K;
    cin >> K;
    set<int> Xs;
    for(int i = 0; i < K; i++) {
        int x;
        cin >> x;
        Xs.insert(x);
    }
    vector<int> st;
    int ans = 0;
    for(int i = 0; i < N - 1; i++) {
        if(Xs.find(i + 1) != Xs.end()) st.push_back(i);
        while(!st.empty() && T - t[i] <= 0) {
            ans++;
            st.pop_back();
            T += 10;
        }
        T -= t[i];
        if(T <= 0) {
            cout << -1 << endl;
            return 0;
        }
    }
    cout << ans << endl;
}