#include"bits/stdc++.h"
#include"atcoder/all"
#define chmin(x,y) (x) = min((x),(y))
#define chmax(x,y) (x) = max((x),(y))
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define vec vector
#define all(a) a.begin(), a.end()
#define pb push_back 

using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;
const ll mod = 998244353;
using mint = modint998244353;
using Graph = vector<vector<pair<int,ll>>>;
const vector<int> dx = {1,0,-1,0}, dy = {0,1,0,-1};

int main(){
	// input
	int N, T;
	cin >> N >> T;
	vector<int> t(N),plus(N,0);
	rep(i,N-1) cin >> t[i+1];
	int K;
	cin >> K;
	rep(i,K){
		int x;
		cin >> x;
		plus[--x] = 1;
	}
	// solve
	int used = 0, stock = 0;
	bool can = 1;
	for(int i = 1; i < N; i++){
		T -= t[i];
		while(T <= 0 && stock){
			stock--;
			T += 10;
			used++;
		}
		if(T <= 0){
			can = 0;
			break;
		}
		
		if(plus[i]) stock++;
	}
	
	// output
	cout << (can ? used : -1) << endl;
}