function airdrop(address[] _receivers, uint256 _amount) public returns (bool) {
    require(block.timestamp > lockups[msg.sender]);
    require(_receivers.length > 0);
    require(_amount > 0);

    uint256 _total = 0;

    for (uint256 i = 0; i < _receivers.length; i++) {
      require(_receivers[i] != address(0));
      require(block.timestamp > lockups[_receivers[i]]);
      _total = _total.add(_amount);
    }

    require(_total <= balances[msg.sender]);

    balances[msg.sender] = balances[msg.sender].sub(_total);

    for (i = 0; i < _receivers.length; i++) {
      balances[_receivers[i]] = balances[_receivers[i]].add(_amount);
      emit Transfer(msg.sender, _receivers[i], _amount);
    }

    return true;
  }