#include<iostream>
#include<cstdio>
using namespace std;
int main(){
    for(;;){
        int a,b,x = 0;
        int field[100] = {0};
        for(;;){
            if(scanf("%d %d",&a,&b) == EOF){
goto jump;
}
            if(!a && !b){break;}
            field[a]++;
            field[b]++;
        }
        for(int i=0; i<100; i++){
            if(field[i]%2 == 1){x++;}
        }
        if(x == 2){cout <<"OK"<<endl;}
        else{cout <<"NG"<<endl;}
    }
    jump:;
    return 0;
}