#include <bits/stdc++.h>

using i64 = long long;

void solve() {
	int n;
	i64 m;
	std::cin >> n >> m;

	std::vector<int> a(n);
	for (int i = 0; i < n; i++) {
		std::cin >> a[i];
	}

	i64 lo = 0, hi = 1E18;

	while (lo < hi) {
		i64 x = (lo + hi + 1) / 2;

		std::vector<i64> b(n);
		for (int i = 0; i < n; i++) {
			b[i] = (x - 1) / a[i] + 1;
		}

		i64 ans = m + 1;

		std::vector<i64> f(n), g(n);
		i64 lst = 0;
		i64 sum = 0;
		for (int i = 0; i < n; i++) {
			g[i] = std::max(0LL, b[i] - 1 - lst);
			f[i] = sum;
			lst = g[i];
			sum += lst;
			sum = std::min(sum, m + 1);
		}
		ans = std::min(ans, 2 * (f[n - 1] + g[n - 1]) + n);
		lst = 0;
		sum = 0;
		for (int i = n - 1; i > 0; i--) {
			i64 v = std::max(0LL, b[i] - lst);
			ans = std::min(ans, 2 * (f[i - 1] + std::max({1LL, g[i - 1], v}) + sum) + i);
			lst = std::max(1LL, v);
			sum += lst;
			sum = std::min(sum, m + 1);
		}
		if (ans <= m) {
			lo = x;
		} else {
			hi = x - 1;
		}
	}

	std::cout << lo << "\n";
}

int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);

	int T;
	std::cin >> T;

	while (T--) {
		solve();
	}

	return 0;
}