#ifndef ONLINE_JUDGE
#define _GLIBCXX_DEBUG
#endif
#include <bits/stdc++.h>
#include <cassert>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using mint = modint998244353;
//using mint = modint1000000007;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repu(i, s, t) for (int i = (int)(s); i < (int)(t); i++)
#define repd(i, s, t) for (int i = (int)(s)-1; i >= (int)(t); i--)
#define all(v) v.begin(), v.end()
void _u() { cerr << endl; } template <class H, class... T> void _u(H&& h, T&&... t) { cerr << h << ", "; _u(move(t)...); }
#define U(...) { cerr << #__VA_ARGS__ << ": "; _u(__VA_ARGS__); }
template<typename T> bool chmax(T &a, const T b) { if(a >= b) return false; a = b; return true; }
template<typename T> bool chmin(T &a, const T b) { if(a <= b) return false; a = b; return true; }
template<typename T> istream& operator>>(istream &in, vector<T> &a) { for(T &x: a) in >> x; return in; }
template<typename T> ostream& operator<<(ostream &out, const vector<T> &a) { for(const T &x: a) out << x << ' '; return out; }
const int di[] = {1, 0, -1, 0, 1, 1, -1, -1, 0};
const int dj[] = {0, 1, 0, -1, -1, 1, 1, -1, 0};


int main() {
    int n, t;
    cin >> n >> t;
    vector<int> ts(n-1);
    cin >> ts;
    int k;
    cin >> k;
    vector<bool> clocks(n);
    rep(i, k) {
        int x;
        cin >> x;
        clocks[x-1] = true;
    }

    int ng = -1, ok = k+1;
    while(ng < ok-1) {
        int m = (ng+ok) / 2;
        int rem = m, cnt = t;
        rep(i, n-1) {
            cnt -= ts[i];
            if(cnt <= 0) break;
            if(rem > 0 && clocks[i+1]) {
                rem--;
                cnt += 10;
            }
        }
        if(cnt <= 0) ng = m;
        else ok = m;
    }
    cout << (ok <= k ? ok : -1) << endl;
    return 0;
}