#include <iostream>
#include <cstdint>
#include <vector>

using namespace std;

static inline constexpr int32_t solve(const uint32_t N, uint32_t T, const uint32_t K, const vector<uint32_t>& t, const vector<uint32_t>& x)
{
	uint32_t stock = 0, used = 0;
	for (uint32_t i = 2, j = 0; i != N + 1; ++i)
	{
		if (t[i] >= T)
		{
			const uint32_t need = (t[i] - T) / 10 + 1;
			if (need > stock) return -1;
			stock -= need, used += need, T = T + need * 10 - t[i];
		}
		else
			T -= t[i];

		if (i == x[j])
			++stock, ++j;
	}

	return used;
}

int main()
{
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	
	uint32_t N, T, K, i;
	cin >> N >> T;
	vector<uint32_t> t(N + 1);
	for (i = 2; i != N + 1; ++i) cin >> t[i];

	cin >> K;
	vector<uint32_t> x(K + 1);
	for (i = 0; i != K; ++i) cin >> x[i];
	x[K] = UINT32_MAX;

	cout << solve(N, T, K, t, x) << '\n';
	return 0;
}