Slight problem. You can cheat by viewing the blockchain: https://blockchain.info/address/18j3TfeSf4MLQsukw7kHiFKj3xhTqfrAD9

Click on the most address sending to that address and you can see how much the person before you sent.

This will resolve itself if people are depositing as frequently as say 1 deposit per 10 seconds. If that doesn't happen then I suggest using a different method.