#include <iostream>
#include <algorithm>
 
using namespace std;
 
int main()
{
  int s, t, a[101];
  bool f;
   
  while(cin>>s>>t){
    fill(a, a+101, 0);
    a[s] = a[t] = 1;
 
    while(cin>>s>>t && s && t){
      a[s]++;
      a[t]++;
    }
 
    f = true;
    if(!(a[1]%2) || !(a[2]%2))
      f = false;
 
    for(int i = 3; i <= 100 && f; i++)
      if(a[i]%2) f = false;
     
    cout << (f? "OK" : "NG") << endl;
  }
   
  return 0;
}