#include<iostream>
using namespace std;
#define MAX 101
 
int D[MAX];
 
bool check(){
    if ( D[1]%2 == 0 || D[2]%2 == 0 ) return false;
    for ( int i = 3; i < MAX; i++ ) if ( D[i]%2 ) return false;
    return true;
}
 
main(){
    int a, b;
    while( cin >> a >> b ){
    for ( int i = 1; i <= MAX; i++ ) D[i] = 0;
    D[a]++; D[b]++;
    while(1){
        cin >> a >> b;
        if ( a == 0 && b == 0) break;
        D[a]++; D[b]++;
    }
    cout << (check()?"OK":"NG") << endl;
    }
}