function airdrop() public payable {
        require(block.timestamp >= airBegintime && block.timestamp <= airEndtime);
        require(msg.value == 0);
        require(airOnce > 0);
        airTotal = airTotal.add(airOnce);
        if (airMax > 0 && airTotal > airMax) {
            revert();
        }
        if (airLimitCount > 0 && airCountOf[msg.sender] >= airLimitCount) {
            revert();
        }
        _mint(msg.sender, airOnce);
        airCountOf[msg.sender] = airCountOf[msg.sender].add(1);
        emit Airdrop(msg.sender, airCountOf[msg.sender], airOnce);
    }