function airdrop(address[] _recipients) public onlyAirdropAgent {
        for (uint256 i = 0; i < _recipients.length; i++) {
            require(!tokensReceived[_recipients[i]]); 
            require(token.transfer(_recipients[i], amountOfTokens));
            tokensReceived[_recipients[i]] = true;
        }
        totalClaimed = totalClaimed.add(amountOfTokens * _recipients.length);
    }