#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r) - 1;i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
template <class T, class F>
T binarySearch(T ok, T ng, const F &f) {
	while (abs(ok - ng) > 1) {
		T mid = (ok + ng) / 2;
		(f(mid) ? ok : ng) = mid;
	}
	return ok;
}
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, m; cin >> n >> m;
	vector<int>t(n - 1);
	rep(i, n - 1)cin >> t[i];
	int k; cin >> k;
	set<int>st;
	rep(i, k) {
		int x; cin >> x;
		st.insert(x - 1);
	}
	auto f = [&](const long long x)->bool {
		int cnt = 0;
		int cm = m;
		rep2(i, 1, n) {
			cm -= t[i - 1];
			if (cm <= 0)return false;
			if (cnt < x && st.count(i)) {
				cnt++;
				cm += 10;
			}
		}
		return true;
	};
	auto ans = binarySearch(k + 1, -1, f);
	if (ans == k + 1)ans = -1;
	cout << ans << endl;
	return 0;
}