#include <bits/stdc++.h>

using namespace std;
#define ll long long
#define pb push_back
int prime[10005];
void sieve()
{
	prime[0] = prime[1] = 1;
	for(int i=2; i*i<=10005; i++) {
		if(prime[i]==0) {
			for(int j=2*i; j<=10005; j+=i) {
				prime[j] = 1;
			}
		}
	}
}

ll stringToN(string s)
{
	int sz = s.size();
	ll num = 0;
	for(int i=0; i<sz; i++) {
		num = num*10 + (s[i]-'0');
	}
	return num;
}

int main()
{
	int t;
	cin >> t;
	int cnt = 1;
	sieve();
	while(t--) {
		int n;
		cin >> n;
		cin.ignore();
		string str;
		cin >> str;
		ll mx = -1;
		for(int i=0; i<n; i++) {
			for(int j=1; j<=10; j++) {
				//cout << str.substr(i, j) << endl;
				//cout << stringToN(str.substr(i, j)) << 	"\n\n";
				ll x = stringToN(str.substr(i, j));
				//cout << x << endl;
				if(x>10000) continue;
				if(!prime[(int)x]) {
					if(mx<x && x<10000) {
						mx = x;
					}
				}
			}
		}
		cout << "Case " << cnt << ": ";
		if(mx==-1) cout << "This is a junk!\n";
		else {
			cout << mx << endl;
		}
		cnt++;
	}
	return 0;
}