#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, t) for (ll i = s; i < (ll)(t); i++)
#define all(x) begin(x), end(x)

template <typename T> bool chmin(T &x, T y) {
	return x > y ? (x = y, true) : false;
}
template <typename T> bool chmax(T &x, T y) {
	return x < y ? (x = y, true) : false;
}

struct IOST {
	IOST() {
		ios::sync_with_stdio(false);
		cin.tie(nullptr);
		cout << fixed << setprecision(20);
	}
} IOST;

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(k);
	rep(i,0,k) cin>>x[i],x[i]--;
	auto check=[&](int mx){
		int ct=t;
		int cx=0;
		rep(i,0,n-1){
			if(ct<=0) break;
			if(cx<mx&&i==x[cx]){
				cx++;
				ct+=10;
			}
			ct-=a[i];
		}
		return ct<=0;
	};
	if(check(k)){
		cout<<"-1\n";
		return 0;
	}
	int up=k;
	int dw=-1;
	while(up-dw>1){
		int md=(up+dw)/2;
		if(check(md)) dw=md;
		else up=md;
	}
	cout<<up<<"\n";
}