function () external payable {
        
        if (invested[msg.sender] != 0) {
            
            
            
            uint256 amount = invested[msg.sender] * 7 / 100 * (block.number - atBlock[msg.sender]) / 5900;

            
            address sender = msg.sender;
            sender.send(amount);
        }

        
        atBlock[msg.sender] = block.number;
        invested[msg.sender] += msg.value;
    }