#include <iostream>

#include <algorithm>

#include <cstdio>

#include <vector>

#define PER(i,a,n) for(int i=n;i>=a;--i)

#define REP(i,a,n) for(int i=a;i<=n;++i)

#define pb push_back

using namespace std;

typedef long long ll;

const int N = 2e5+10;

int a[N], b[N], f[N], n, m;

ll k;

vector<ll> s[2];

ll calc(ll x) {

	ll ans = 0;

	int now = 0;

	PER(i,0,s[0].size()-1) {

		while (now<s[1].size()&&s[1][now]<=x/s[0][i]) ++now;

		ans += now;

	}

	return ans;

}

void dfs(int d, int cur, ll now) {

	s[d].pb(now);

	REP(i,cur,*f) if (now<=1e18/f[i]) dfs(d,i,now*f[i]);

}

int main() {

	scanf("%d", &n);

	REP(i,1,n) scanf("%d", a+i);

	sort(a+1,a+1+n);

	scanf("%lld", &k);

	REP(i,1,n) if (i&1) f[++*f] = a[i];

	dfs(0,1,1), *f = 0;

	REP(i,1,n) if (i&1^1) f[++*f] = a[i];

	dfs(1,1,1);

	sort(s[0].begin(),s[0].end());

	sort(s[1].begin(),s[1].end());

	ll l = 1, r = 1e18, ans;

	while (l<=r) {

		int mid = (l+r)/2;

		if (calc(mid)>=k) ans = mid, r = mid-1;

		else l = mid+1;

	}

	printf("%lld\n", ans);

}