#include <iostream>
#include <string>
#include <algorithm>

using namespace std;

bool compare(string s1, string s2)
{
	transform(s1.begin(), s1.end(), s1.begin(), ::toupper);
	transform(s2.begin(), s2.end(), s2.begin(), ::toupper);
	return s1 == s2;
}

int main() {
	
	string s[] = {"sTAck", "STACk", "STAcK", "SSTTAACCKK", "stack", "Extack"};
	
	for(int i = 0; i < 6; i++)
		cout << s[i] << (compare(s[i], "stack") ? " OK" : " NOK") << endl;
	
	return 0;
}