#include <bits/stdc++.h>

using i64 = long long;

constexpr int N = 1E5;

void solve() {
    int n, c;
    std::cin >> n >> c;
    
    std::bitset<N + 1> a{}, b{};
    for (int i = 0; i < n; i++) {
        int x;
        std::cin >> x;
        a[x] = b[N - x] = 1;
    }
    
    double ans = -1;
    
    std::vector<int> lst(2 * c + 1);
    for (int i = 1; i <= 2 * c; i++) {
        lst[i] = i <= N && a[i] ? i : lst[i - 1];
    }
    
    for (int s = 2 * c; s >= 3; s--) {
        int z = lst[s - 1];
        if (2 * z >= s) {
            auto v = a & (s < N ? b >> (N - s) : b << (s - N));
            int x = v._Find_next(s - z);
            int y = s - x;
            if (x < y && y < z) {
                double p = 0.5 * (x + y + z);
                double area = std::sqrt(p * (p - x) * (p - y) * (p - z));
                if (ans < 0 || ans > area) {
                    ans = area;
                }
            }
        }
    }
    for (int z = 1; z <= c; z++) {
        if (a[z]) {
            int y = lst[z - 1];
            if (!y) {
                continue;
            }
            int x = a._Find_next(z - y);
            if (x < y) {
                double p = 0.5 * (x + y + z);
                double area = std::sqrt(p * (p - x) * (p - y) * (p - z));
                if (ans < 0 || ans > area) {
                    ans = area;
                }
            }
        }
    }
    
    std::cout << ans << "\n";
}

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    
    std::cout << std::fixed << std::setprecision(10);
    
    int t;
    std::cin >> t;
    
    while (t--) {
        solve();
    }
    
    return 0;
}