function drawRandomWinner() public onlyAdmin {
    require(raffleEndTime < block.timestamp); 
    require(!raffleWinningTicketSelected);
        
    uint256 seed = SafeMath.add(raffleTicketsBought , block.timestamp);
    raffleTicketThatWon = addmod(uint256(block.blockhash(block.number-1)), seed, raffleTicketsBought);
    raffleWinningTicketSelected = true;
  }