#include <bits/stdc++.h>
#define ll long long
#define int long long
using namespace std;

const int maxn = 110000;

int n,C;
int a[maxn];

long double calc(long double x,long double y,long double z)
{
	long double p=(x+y+z)/2;
	return sqrtl( p*(p-x)*(p-y)*(p-z) );
}

signed main()
{
	ios_base::sync_with_stdio(false); ////////////////////////////////////////
	cin.tie(0);
	
	int Tcase; cin>>Tcase;
	while(Tcase--)
	{
		cin>>n>>C;
		for(int i=1;i<=n;i++) cin>>a[i];
		sort(a+1,a+n+1);
		
		long double ans=1e15;
		for(int i=2;i<n;i++)
		{
			int l=1,r=i-1;
			while(l<=r)
			{
				int mid=(l+r)>>1;
				if(a[mid]+a[i]<=a[i+1]) l=mid+1;
				else r=mid-1;
			}r++;
			
			if(r<i) ans=min(ans,calc(a[r],a[i],a[i+1]));
		}
		
		for(int i=1;i<n;i++)
		{
			int k=0;
			for(int j=i+1;j<n;j++)
			{
				if(k<j+1) k=j+1;
				while(k+1<=n && a[i]+a[j]>a[k+1]) k++;
				
				if(a[i]+a[j]>a[k])
				{
					ans=min(ans, calc(a[i],a[j],a[k]));
					break;
				}
			}
		}
		
		if(ans>1e12) cout<<-1<<'\n';
		else cout<<fixed<<setprecision(15)<<ans<<'\n';
	}
	
	return 0;
}