#include <bits/stdc++.h>
using namespace std;
#define repl(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define rep(i,n) repl(i,0,n)
#define mp(a,b) make_pair((a),(b))
#define pb(a) push_back((a))
#define all(x) (x).begin(),(x).end()
#define uniq(x) sort(all(x)),(x).erase(unique(all(x)),end(x))
#define fi first
#define se second
#define dbg(x) cout<<#x" = "<<((x))<<endl
template<class T,class U> ostream& operator<<(ostream& o, const pair<T,U> &p){o<<"("<<p.fi<<","<<p.se<<")";return o;}
template<class T> ostream& operator<<(ostream& o, const vector<T> &v){o<<"[";for(T t:v){o<<t<<",";}o<<"]";return o;}

#define INF 2147483600
#define long long long // for codeforces

int main(){
  int a,b;
  while(cin>>a>>b){
    vector<int> vec(100,0);
    do{
      a--;b--;
      vec[a]++;
      vec[b]++;
    } while(cin>>a>>b, a|b);

    vec[0]++; vec[1]++;
    bool ok=true;
    rep(i,100) if(vec[i]%2==1) ok=false;
    if(ok) cout<<"OK"<<endl;
    else cout<<"NG"<<endl;
  }

  return 0;
}