#include <iostream>
#include <string>
using namespace std;

int main() {
  string a, b;
  cin >> a >> b;

  if ((a.size()!=1 && a[0]=='0') || (b.size()!=1 && b[0]=='0')) {
    cout << "NG" << endl;
    return 0;
  } else {
    for (int i=0; i<a.size(); i++) {
      if (a[i]-'0'>i+1) {
        cout << "NG" << endl;
        return 0;
      }
    }
    for (int i=0; i<b.size(); i++) {
      if (b[i]-'0'>i+1) {
        cout << "NG" << endl;
        return 0;
      }
    }
  }

  cout << "OK" << endl;
}