#include<iostream>
#include<vector>
#include<algorithm>
#include<iomanip>
using namespace std;
#define OVERLOAD_REP(_1, _2, _3, name, ...) name
#define REP1(i, n) for (auto i = std::decay_t<decltype(n)>{}; (i) != (n); ++(i))
#define REP2(i, l, r) for (auto i = (l); (i) != (r); ++(i))
#define rep(...) OVERLOAD_REP(__VA_ARGS__, REP2, REP1)(__VA_ARGS__)
#define REP(i, l, r) rep(i, l, r+1)
#define all(x) (x).begin(),(x).end()
#define inf 1000000000
using ll = long long;
using P = pair<int,int>;
using Graph = vector<vector<int> >;

int main() {
    // 高速化
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    // 小数点の出力桁数を指定
    cout << fixed << setprecision(10);

    // メイン
    int N, T, K;
    cin >> N >> T;
    vector<int> t(N-1);
    rep(i, 0, N-1) cin >> t[i];
    cin >> K;
    vector<int> x(K);
    rep(i, 0, K) cin >> x[i];
    x.push_back(N);
    rep(i, 0, K+1) x[i]--;
    vector<int> ruiseki(N-1);
    ruiseki[0] = t[0];
    rep(i, 1, N-1) ruiseki[i] = ruiseki[i-1] + t[i];
    vector<int> times(K+1);
    rep(i, 0, K+1) {
        if(i == 0) times[i] = ruiseki[x[i]-1];
        else times[i] = ruiseki[x[i]-1] - ruiseki[x[i-1]-1];
    }

    int ans = 0;
    rep(i, 0, K+1) {
        T-=times[i];
        if(T <= 0) {
            cout << -1 << endl;
            return 0;
        }
        if(i != K && T <= times[i+1]) {
            ans++;
            T += 10;
        }
    }
    cout << ans << endl;

    return 0;
}