Copy the server key from: https://console.firebase.google.com/project/cs5520a7-301d3/settings/cloudmessaging/android:edu.neu.madcourse.cs5520_a7 into app/src/main/assets/firebase.properties

The app/src/main/assets/firebase.properties should contians one line: SERVER_KEY: AAAA8...AKl8

• If it is the first time using this app, please click login button to enter a username in order to register the username into the database
• If you would like to send a sticker to a certain user, please click send sticker button, then enter a username, choose a sticker and click the send button to send a certain sticker to a specific user
• If you would like to see what stickers you have received in the past, please click history button to see the stickers received so far