pragma solidity >=0.5.11;

// ERC Token Standard #20 Interface
// https://github.com/ethereum/EIPs/issues/20
contract CYT {
    string public constant symbol = "CYT";
    string public constant name = "crystal token";
    uint8 public constant decimals = 18;
    uint256 _totalSupply = 21 * (10 ** 8) * (10 ** 18);

    address public owner;

  
    mapping(address => uint256) balances;
    
    mapping(address => mapping (address => uint256)) allowed;

    constructor() public {
        owner = msg.sender;
        balances[owner] = _totalSupply;
    }

    function () external {
        revert("Send back ether sent to me");
    }

    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address _owner) public view returns (uint256 balance) {
        return balances[_owner];
    }

    function transfer(address _to, uint256 _amount) public returns (bool success) {
        if (balances[msg.sender] >= _amount && _amount > 0 && balances[_to] + _amount > balances[_to]) {
            balances[msg.sender] -= _amount;
            balances[_to] += _amount;
            emit Transfer(msg.sender, _to, _amount);
            return true;
        } else {
            return false;
        }
    }

  
    function transferFrom(address _from, address _to, uint256 _amount) public returns (bool success) {
        if (balances[_from] >= _amount && allowed[_from][msg.sender] >= _amount && _amount > 0 && balances[_to] + _amount > balances[_to]) {
            balances[_from] -= _amount;
            allowed[_from][msg.sender] -= _amount;
            balances[_to] += _amount;
            emit Transfer(_from, _to, _amount);
            return true;
        } else {
            return false;
        }
    }


    function approve(address _spender, uint256 _amount) public returns (bool success) {
        allowed[msg.sender][_spender] = _amount;
        emit Approval(msg.sender, _spender, _amount);
        return true;
    }

    
    function allowance(address _owner, address _spender) public view returns (uint256 remaining) {
        return allowed[_owner][_spender];
    }

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}