#include <bits/stdc++.h>
using namespace std;

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int N,T; cin >> N >> T;
    vector<int> A(N-1);
    for(auto &t : A) cin >> t;

    vector<bool> X(N);
    int K; cin >> K;
    while(K--){
        int p; cin >> p; p -= 2;
        X.at(p) = true; 
    }

    int have = 0,answer = 0,idx = 0;
    for(auto t : A){
        T -= t;
        while(T <= 0){
            if(have == 0){answer = -1; break;}
            have -= 1; answer += 1; T += 10;
        }
        if(answer == -1) break;
        if(X.at(idx)) have++;
        idx++;
    }
    cout << answer << endl;
}