function flight(agent) {
    const city = agent.parameters['geo-city'];
    const time = agent.parameters['time'];
    const gotCity = city.length > 0;
    const gotTime = time.length > 0;

    if(gotCity && gotTime) {
        agent.add(`Nice, you want to fly to ${city} at ${time}.`);
    } else if (gotCity && !gotTime) {
        agent.add('Let me know which time you want to fly');
    } else if (gotTime && !gotCity) {
        agent.add('Let me know which city you want to fly to');
    } else {
        agent.add('Let me know which city and time you want to fly');
    }
  }