#include<iostream>
#include<vector>
using namespace std;

int main()
{
	int N,T;
	cin>>N>>T;
	vector<int> t(N-1);
	for(int &i:t)cin>>i;
	int K;
	cin>>K;
	bool k[1<<18];
	for(int i=0;i<K;++i)
	{
		int num;
		cin>>num;
		k[num-1] = true;
	}
	
	int req = 1;
	for(int i=0;i<N-1;++i)req += t[i];
	int times = (req-T+9) / 10;
	int ans = times;
	if(req < T) ans = 0;
	for(int i=0;i<N-1;++i)
	{
		T -= t[i];
		if(T<=0)
		{
			cout << -1 << endl;
			return 0;
		}
		if(k[i] && times--)T+=10;
	}

	cout << ans << endl;
}