#include<bits/stdc++.h>
using namespace std;
vector<vector<int>> edge;
int input(){
edge.clear();
edge.resize(110);
while(1){
int a,b;
if(!(cin>>a>>b))return false;
if(a==0)break;
a--;b--;
edge[a].push_back(b);
edge[b].push_back(a);
}
return true;
}
bool solve(){
for(int i=2;i<edge.size();i++){
if(edge[i].size()%2!=0){
return false;
}
}
if(edge[0].size()%2!=1)return false;
if(edge[1].size()%2!=1)return false;
return true;
}
int main(){
while(input()){
if(solve()){
cout<<"OK"<<endl;
}else cout<<"NG"<<endl;
}
}