
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<math.h>
#include<string>
#include<string.h>
#include<stack>
#include<queue>
#include<vector>
#include<utility>
#include<set>
#include<map>
#include<stdlib.h>
#include<iomanip>

using namespace std;

#define ll long long
#define ld long double
#define EPS 0.0000000001
#define INF 1e9
#define MOD 1000000007
#define rep(i,n) for(i=0;i<n;i++)
#define loop(i,a,n) for(i=a;i<n;i++)
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)

typedef vector<int> vi;
typedef pair<int,int> pii;

int main(void) {
  int i,j;
  int a,b;
  while(cin>>a>>b){
    vi num(100,0);
    num[a-1]++;
    num[b-1]++;
    while(a+b>0){
      cin>>a>>b;
      num[a-1]++;
      num[b-1]++;
    }
    bool ans=true;
    if(num[0]%2==0 || num[1]%2==0){
      cout<<"NG"<<endl;
      ans=false;
    }
    loop(i,2,100)if(num[i]%2){
      cout<<"NG"<<endl;
      ans=false;
      break;
    }
    if(ans)cout<<"OK"<<endl;
  }
}