#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,s,n) for (int i = (s); i < (n); ++i)
#define rrep(i,n,g) for (int i = (n)-1; i >= (g); --i)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define len(x) (int)(x).size()
#define dup(x,y) (((x)+(y)-1)/(y))
#define pb push_back
#define eb emplace_back
#define Field(T) vector<vector<T>>
#define pq(T) priority_queue<T, vector<T>, greater<T>>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using P = pair<int,int>;
template<class T>bool chmax(T&a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T>bool chmin(T&a,T b){if(b<a){a=b;return 1;}return 0;}

int main() {
  int n, t;
  cin >> n >> t;
  vector<int> a(n-1);
  rep(i,0,n-1) cin >> a[i];
  int k;
  cin >> k;
  vector<int> x(n, 0);
  rep(i,0,k) {
    int xi;
    cin >> xi;
    x[xi-1] = 1;
  }
  function<bool(int)> f = [&](int c) {
    int v = t;
    int cnt = 0;
    rep(i,0,n-1) {
      v -= a[i];
      if (v <= 0) return false;
      if (x[i+1] && cnt < c) {
        v += 10;
        ++cnt;
      }
    }
    return true;
  };
  // rep(i,0,k+1) {
  //   cout << f(i) << " ";
  // }
  // cout << endl;
  if (!f(k)) {
    cout << -1 << endl;
    return 0;
  }
  int ng = -1, ok = k;
  while(ok-ng > 1) {
    int mid = (ok+ng)/2;
    if (f(mid)) ok = mid;
    else ng = mid;
  }
  cout << ok << endl;
  return 0;
}