#include "bits/stdc++.h"

using namespace std;

using ll = long long;
using ld = long double;

const double PI = 3.1415926535897932384626433832795;
const ll MOD = 1000000007;

const int dx[] = { 0, 1, 0, -1 };
const int dy[] = { -1, 0, 1, 0 };

int gcd(int x, int y) { return y ? gcd(y, x % y) : x; }
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
int lcm(int x, int y) { return x / gcd(x, y) * y; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }

const int MAX_N = 101;

bool solve(vector<int> &deg) {
	bool isBreak = false;
	for (int i = 2; i < MAX_N; i++) {
		if (deg[i] & 1) {
			return false;
		}
	}

	for (int i = 0; i < 2; i++) {
		if (deg[i] % 2 == 0) {
			return false;
		}
	}

	return true;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	int a, b;
	while (true) {
		vector<int> deg(MAX_N);
		int a = -1333;
		while (cin >> a >> b) {
			a--; b--;
			if (a == -1 && b == -1) {
				break;
			}

			// a -> b
			deg[a]++;
			deg[b]++;
		}
		if (a == -1333) {
			return 0;
		}

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

	return 0;
}

