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

int main() {
  int N, T, K;
  cin >> N >> T;
  vector<int> t(N - 1, 0), sum(N, 0);
  for(int i = 0; i < N - 1; i++) {
    cin >> t[i], sum[i + 1] = sum[i] + t[i];
  }
  cin >> K;
  vector<int> x(K, 0);
  for(int &o : x) {
    cin >> o, o--;
  }
  for(int i = 0; i < K; i++) {
    if(T + 10 * i <= sum[x[i]]) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << max((sum[N - 1] - T) / 10, -1) + 1 << endl;
}