#include<bits/stdc++.h>
using namespace std;
int t, n, m, a[50005], ans;
int S(int p, int q, int w) {
	double e = (p + q + w) / 2.0;
	return	round(sqrt(e * (e - p) * (e - q) * (e - w)));
}
bitset<100005> b;
int readNum() {
	int x = 0, f = 1;
	char ch = getchar();
	while (ch < '0' || ch > '9') {
		if (ch == '-') f = -1;
		ch = getchar();
	}
	while (ch >= '0' && ch <= '9') {
		x = (x << 1) + (x << 3) + (ch ^ 48);
		ch = getchar();
	}
	return x * f;
}
int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	t = readNum();
	while (t--) {
		n = readNum();
		m = readNum();
		for (int i = 1; i <= n; i++) {
			a[i] = readNum();
		}
		if (n == m) {
			if (n <= 3) {
				cout << -1 << endl;
			} else {
				cout << 3 << endl;
			}
			continue;
		}
		sort(a + 1, a + n + 1);
		b.reset();
		bool flg = true;
		for (int i = 3; i <= n; i++) {
			if (a[i] < a[i - 1] + a[i - 2]) {
				flg = false;
				break;
			}
		}
		if (flg) {
			cout << -1 << endl;
			continue;
		}
		for (int i = 1; i <= n; i++) {
			b.set(a[i], 1);
		}
		bitset<100005> t;
		ans = 0x3f3f3f3f;
		for (int d = 1; d <= m / 2; d++) {
			int q = (b >> (d + 1))._Find_first();
			//			cout << q << ' ';
			if (q == -(1 << 31) - 1) continue;
			q += d + 1;
			t = b & (b >> d);
			t >>= (q + 1);
			if (t.count() == 0) continue;
			int p = t._Find_first();
			ans = min(ans, S(p + q + 1, p + q + d + 1, q ));
			//			cout << p << ' ' << q << ' ' << d << endl;
		}
		cout << ans << endl;
	}
	return 0;
}