#include <bits/stdc++.h>
#include <cassert>

using namespace std;

typedef long long ll;
typedef long double ld;

// read template
long long readInt(long long l,long long r,char endd){
	long long x=0;
	int cnt=0;
	int fi=-1;
	bool is_neg=false;
	while(true){
		char g=getchar();
		if(g=='-'){
			assert(fi==-1);
			is_neg=true;
			continue;
		}
		if('0'<=g && g<='9'){
			x*=10;
			x+=g-'0';
			if(cnt==0){
				fi=g-'0';
			}
			cnt++;
			assert(fi!=0 || cnt==1);
			assert(fi!=0 || is_neg==false);
			
			assert(!(cnt>19 || ( cnt==19 && fi>1) ));
		} else if(g==endd){
			assert(cnt>0);
			if(is_neg){
				x= -x;
			}
			assert(l<=x && x<=r);
			return x;
		} else {
			assert(false);
		}
	}
}

string readString(int l,int r,char endd){
	string ret="";
	int cnt=0;
	while(true){
		char g=getchar();
		assert(g!=-1);
		if(g==endd){
			break;
		}
		cnt++;
		ret+=g;
	}
	assert(l<=cnt && cnt<=r);
	return ret;
}

long long readIntSp(long long l,long long r){
	return readInt(l,r,' ');
}

long long readIntLn(long long l,long long r){
	return readInt(l,r,'\n');
}

string readStringLn(int l,int r){
	return readString(l,r,'\n');
}

string readStringSp(int l,int r){
	return readString(l,r,' ');
}
// end

const int M = 1e5 + 239;
const int th = 3;

int n, k, a[M], sum, bef[th][M];
map<int, int> kol;

void solve()
{
	n = readIntSp(1, (int)(1e5));
	k = readIntLn(0, n);	
	for (int i = 0; i < n; i++)
	{
		if (i < n - 1)
			a[i] = readIntSp(1, (int)(1e9));
		else
			a[i] = readIntLn(1, (int)(1e9));
	}
	sum += n;
	kol.clear();
	for (int i = 0; i < n; i++) kol[a[i]]++;
	k -= kol.size();
	if (k <= 0)
	{
		cout << "-1\n";
		return;
	}
	int mx = 0;
	for (pair<int, int> t : kol) mx = max(mx, t.second - 1);
	if (k <= min(mx, 2))
	{
		cout << "-1\n";
		return;
	}
	vector<pair<int, int> > now;
	for (pair<int, int> t : kol) 
		if (t.second >= 2)
			now.push_back(make_pair(t.second - 1, t.first));
	ll ans = (ll)(1e18) + 239;
	int kl = 0;
	for (int x = 0; x < 3; x++)
	{
		bef[x][now.size()] = now.size();
		for (int i = (int)now.size() - 1; i >= 0; i--)
		{
			bef[x][i] = bef[x][i + 1];
			if (now[i].first >= x) 
				bef[x][i] = i;
		}
	}
	for (int i = 0; i < (int)now.size(); i++)
	{
		kl += now[i].first;
		if (now[i].second >= 3 && kl >= k) ans = min(ans, (ll)now[i].second * (ll)now[i].second);
		if (k - kl > 2) continue;
		if (i + 1 >= now.size()) continue;
		int it = bef[max(k - kl, 0)][i + 1];
		if (it < now.size() && k - kl <= 2) ans = min(ans, (ll)now[i].second * (ll)now[it].second);
	}		
	cout << ans << "\n";
}

int main()
{ 
	int T = readIntLn(1, 100);
	sum = 0;
	while (T--) solve();		
	assert(getchar() == -1);	
	assert((sum <= (int)(1e5)));	
	return 0;
}