#include <bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int N, T, K;
    cin >> N >> T;
    vector<int> t(N - 1);
    for (int i = 0; i < N - 1; i++) cin >> t[i];
    
    cin >> K;
    unordered_set<int> clockLocations;
    for (int i = 0; i < K; i++) {
        int x; cin >> x;
        clockLocations.insert(x - 1); // Store as 0-based index
    }
    
    deque<int> dq; // Deque to store available clocks efficiently
    int timeLeft = T, clocksUsed = 0;
    
    for (int i = 0; i < N - 1; i++) {
        timeLeft -= t[i]; // Move to the next location
        if (timeLeft < 0) { // If time runs out before reaching
            cout << "-1\n";
            return 0;
        }
        if (clockLocations.count(i)) dq.push_back(10); // Add +10 clock if available
        
        while (!dq.empty() && timeLeft < 0) { // Use clocks only when absolutely necessary
            timeLeft += dq.front();
            dq.pop_front();
            clocksUsed++;
        }
        
        if (timeLeft < 0) { // If we still can't proceed, fail
            cout << "-1\n";
            return 0;
        }
    }
    
    cout << clocksUsed << "\n";
    return 0;
}