#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;
  }
  bool f = false;
  int tmp = T;
  for(int i = 0; i < K; i++) {
    if(tmp + 10 * i <= sum[x[i]]) {
      f = true;
    }
  }
  if(f) {
    cout << -1 << endl;
    return 0;
  }
  int l = -1, r = K, c = 0;
  while(r - l > 1) {
    c = (l + r) / 2;
    if(tmp + 10 * c <= sum.at(N - 1)) {
      l = c;
    }
    else {
      r = c;
    }
  }
  cout << r << endl;
}