CVBB ACM Team

#include<cstdio>
#include<cassert>
#include<iostream>
#include<algorithm>
#include<vector>
#define pb push_back
using namespace std;
typedef long long LL;
const int N=2e5+5;
const LL INF=1e18;
vector<LL>pos,neg;
int main()
{
	int n;
	LL k;
	scanf("%d%lld",&n,&k);
	for(int i=1;i<=n;i++)
	{
		int a;
		scanf("%d",&a);
		if(a>0)pos.pb(a);
		else if(a<0)neg.pb(a);
	}
	sort(pos.begin(),pos.end());
	sort(neg.begin(),neg.end());
	LL ne=1LL*neg.size()*pos.size();
	LL po=1LL*pos.size()*(pos.size()-1)/2+1LL*neg.size()*(neg.size()-1)/2;
	LL ze=1LL*n*(n-1)/2-ne-po;
	if(k<=ne)
	{
		LL l=-INF,r=0,ans;
		while(l<=r)
		{
			LL mid=(l+r)/2,num=0;
			for(int i=0;i<neg.size();i++)
			{
				LL t=mid%neg[i]?mid/neg[i]+1:mid/neg[i];
				int x=lower_bound(pos.begin(),pos.end(),t)-pos.begin();
				num+=pos.size()-x;
			}
			if(num<k)l=mid+1;
			else r=mid-1,ans=mid;
		}
		cout<<ans;
	}
	else if(k<=ne+ze)puts("0");
	else
	{
		LL l=0,r=INF,ans;
		while(l<=r)
		{
			LL mid=(l+r)/2,num=0;
			for(int i=0;i<pos.size();i++)
			{
				if(pos[i]*pos[i]<=mid)num--;
				LL t=mid/pos[i];
				int x=lower_bound(pos.begin(),pos.end(),t+1)-pos.begin();
				if(x)assert(pos[x-1]*pos[i]<=mid);
				if(x<pos.size())assert(pos[x]*pos[i]>mid);
				num+=x;
			}
			for(int i=0;i<neg.size();i++)
			{
				if(neg[i]*neg[i]<=mid)num--;
				LL t=mid/neg[i];
				int x=lower_bound(neg.begin(),neg.end(),t)-neg.begin();
				assert(neg[x]*neg[i]<=mid);
				if(x)assert(neg[x-1]*neg[i]>mid);
				num+=neg.size()-x;
			}
			assert(num%2==0);
			num=num/2+ze+ne;
			if(num<k)l=mid+1;
			else r=mid-1,ans=mid;
		}
		cout<<ans;
	}
	return 0;
}