#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;
    vector<int> clocks(K);
    for (int i = 0; i < K; i++) {
        cin >> clocks[i];
        clocks[i]--; // Convert to 0-based index
    }
    
    sort(clocks.begin(), clocks.end()); // Process clocks in order
    priority_queue<int, vector<int>, greater<int>> pq; // Min-heap for +10 clocks
    
    int timeLeft = T, clocksUsed = 0, clockIndex = 0;
    
    for (int i = 0; i < N - 1; i++) {
        timeLeft -= t[i]; // Move to the next location

        // If current location has a clock, add it to the available pool
        while (clockIndex < K && clocks[clockIndex] == i) {
            pq.push(10);
            clockIndex++;
        }

        // Use clocks before running out of time
        while (timeLeft < 0 && !pq.empty()) {
            timeLeft += pq.top();
            pq.pop();
            clocksUsed++;
        }

        if (timeLeft < 0) { // If we still can't proceed, fail
            cout << "-1\n";
            return 0;
        }
    }
    
    // If we reach the goal with exactly 0 time left, we must use a clock
    if (timeLeft == 0 && !pq.empty()) {
        timeLeft += pq.top();
        pq.pop();
        clocksUsed++;
    }
    
    if (timeLeft <= 0) { // Ensure we didn't fail at the goal
        cout << "-1\n";
    } else {
        cout << clocksUsed << "\n";
    }
    
    return 0;
}