#include <iostream>
using namespace std;
 
int main(){
    string A, B;
    cin >> A >> B;
    if((A[0] == '0' && A.size() != 1) || (B[0] == '0' && B.size() != 1)){
        cout << "NG" << endl;
        return 0;
    }
    for(int i = 0; i < A.size(); i++){
        if(A[i] < '0' || A[i] > '1' + i){
            cout << "NG" << endl;
            return 0;
        }
    }
    for(int i = 0; i < B.size(); i++){
        if(B[i] < '0' || B[i] > '1' + i){
            cout << "NG" << endl;
            return 0;
        }
    }
    cout << "OK" << endl;
}