/***
 * Solution to Liars and Saints - CodeIT Season 6, Round 6, Problem J2
 * Author - Bozhidar Vasilev
**/

#include <iostream>
#include <string>

using namespace std;

int main()
{
    int n;
    cin >> n;
    string s;
    cin >> s;
    bool good = true;
    for( char i : s )
    {
        if( i != 'L' )
            good = false;
    }
    if( good )
    {
        cout << "OK" << endl;
    }
    else
    {
        cout << "INVALID" << endl;
    }
    return 0;
}