#include<bits/stdc++.h>
using namespace std;
#define rep(i,N) for(i=0;i<N;i++)
#define ll long long

int main(void){
    ll N,T;
    ll t[200009]={0};
    bool tokei[200009]={0};
    ll K;

    ll i,j,k;
    
    cin>>N>>T;
    rep(i,N-1)cin>>t[i+1];
    cin>>K;
    rep(i,K){
        cin>>j;
        tokei[j]=1;
    }

    ll now=T;
    ll have=0;
    ll ans=1;
    for(i=1;i<N;i++){
        if(now-t[i]+10*have<=0){
            ans=0;
            break;
        }
        now-=t[i];
        if(tokei[i+1])have++;
    }

    if(ans==0)ans=-1;
    else{
        ans=0;
        while(now<=0){
            ans++;
            now+=10;
        }
    }

    cout<<ans<<endl;

    return 0;
}