Entrance.sol

pragma solidity >=0.4.21 <0.6.0;

//Code from 35C3 Junior CTF

import "./SafeMath.sol"; //https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/math/SafeMath.sol

contract Entrance {
  using SafeMath for *;
  mapping(address => uint256) public balances;
  mapping(address => bool) public has_played;
  uint256 pin;

  event EntranceFlag(string server, string port);
  event Win();

  modifier legit(uint256 _pin) {
    if (_pin == pin) _;
  }

  modifier onlyNewPlayer {
    if (has_played[msg.sender] == false) _;
  }

  constructor(uint256 _pin) public {
    pin = _pin;
  }

  function enter(uint256 _pin) public legit(_pin) {
    balances[msg.sender] = 10;
    has_played[msg.sender] = false;
  }

  function balanceOf(address _who) public view returns (uint256 balance) {
    return balances[_who];
  }

  function gamble() public onlyNewPlayer {
    require (balances[msg.sender] >= 10);
    if ((block.number).mod(7) == 0) {
      balances[msg.sender] = balances[msg.sender].add(10);
      // Tell the sender he won!
      msg.sender.call("You won!");
      has_played[msg.sender] = true;
    } else {
      balances[msg.sender] = balances[msg.sender].sub(10);
    }
  }

  function getFlag(string memory _server, string memory _port) public {
    require (balances[msg.sender] > 300);
    emit EntranceFlag(_server, _port);
  }
}