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

int main()
{
    int n, m;
    cin >> n >> m;
    vector<int> t(n - 1);
    long long sum = 0;
    for (int i = 0; i < n - 1; i++)
    {
        cin >> t[i];
    }
    int k;
    cin >> k;
    vector<int> x(k);
    vector<int> y(n, 0);
    for (int i = 0; i < k; i++)
    {
        cin >> x[i];
        y[x[i]] = 1;
    }
    long long cnt = 0;
    queue<int> q;
    for (int i = 0; i < n - 1; i++)
    {
        m -= t[i];
        if (y[i] == 1)
        {
            q.push(i);
        }
        if (m <= 0)
        {
            while (!q.empty() && m <= 0)
            {
                q.pop();
                m += 10;
                cnt++;
            }
            if (m <= 0)
            {
                cout << -1 << endl;
                return 0;
            }
        }
    }
    cout << cnt << endl;
}