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

#define rep(i, n) for (int i = 0; i < (int)n; i++)

int main() {
  int n, t;
  cin >> n >> t;
  vector<int> ti(n, 0), xi(n, 0);
  rep(i, n-1) cin >> ti[i];
  int k;
  cin >> k;
  rep(i, k) {
    int x; cin >> x;
    xi[x-1] = 10;
  }
  t -= ti[0];
  int cnt = 0;
  int res = 0;
  rep(i, n-1) {
    res += xi[i];
    if (t - ti[i+1] <= 0) {
      while (res > 0 && t <= ti[i+1]) {
	t += 10;
	cnt++;
	res -= 10;
      }
    }
    t -= ti[i+1];
    if (t < 0) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << cnt << endl;
  return 0;
}