#include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <climits>
#include <cmath>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <algorithm>
#include <functional>

using namespace std;

typedef long long ll;

#define ITER( c ) __typeof( c ).begin()
#define REP( i, m, n ) for ( int i = m; i < n; i++ )
#define UREP( i, m, n ) for ( unsigned int i = m; i < n; i++ )
#define IREP( c, it ) for ( ITER(c) it = c.begin(); it != c.end(); ++it )
#define ALL( c ) c.begin(), c.end()
#define PB( n ) push_back( n );
#define MP( a, b ) make_pair( a, b )
#define fst first
#define snd second

int main()
{
	map<int,int> inter;

	int m, n;
	while ( cin >> m >> n )
	{
		inter[m]++;
		inter[n]++;

		while ( true )
		{
			cin >> m >> n;

			if ( m == 0 && n == 0 )
			{
				break;
			}

			inter[m]++;
			inter[n]++;
		}

		bool valid = true;

		IREP( inter, it )
		{
			if ( 3 <= (*it).first && (*it).second % 2 )
			{
				valid = false;
			}
		}

		cout << ( valid ? "OK" : "NG" ) << endl;
	}

	return 0;
}