#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define ll long long
#define INF 9e18;
int main(){
    cin.tie(0)->sync_with_stdio(0);
    cout.tie(0);
    ll N,T,K;
    cin>>N>>T;
    ll t[200009],x[200009];
    for(int i=1;i<=N-1;i++) cin>>t[i];
    cin>>K;
    for(int i=1;i<=K;i++) cin>>x[i];
    ll plus[200009];
    for(int i=1;i<=K;i++) plus[x[i]]=1;
    ll tyokin=0;
    ll ans=0;
    for(int i=1;i<=N-1;i++){
        T-=t[i];
        if(T<=0){
            while(T<=0){
                T+=10;
                tyokin--;
                ans++;
            }
        }
        if(tyokin<0){
            cout<<-1<<endl;
            return 0;
        }
        tyokin+=plus[i];
    }
    cout<<ans<<endl;
}