#include <vector>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <algorithm>
#include <utility>
#include <functional>
#include <sstream>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <climits>
using namespace std;
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v;}
template<class T> inline string toStr(T x) { ostringstream sout; sout << x; return sout.str();}
typedef vector<int> vi;
typedef vector<vi>  vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define FOR(i,a,b) for(int i=(a);i<=(b);++i)
#define REP(i,n) FOR(i,0,(n)-1)
const double EPS = 1e-12;
const double PI = acos(-1.0);
const int INF = INT_MAX/10;

int main() {
	int a, b;
	while(cin >> a >> b) {
		vi num;
		num.push_back(a);
		num.push_back(b);

		while(cin >> a >> b, a|b) {
			num.push_back(a);
			num.push_back(b);
		}

		int size = num.size();
		int m = 0;
		REP(i, size) {
			m = max(m, num[i]);
		}
		vi res(m+1);
		REP(i, size) {
			res[num[i]]++;
		}
		bool flag = false;
		FOR(i, 3, m) {
			if(res[i]%2) {
				flag = true;
			}
		}

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

	return 0;
}