contract StandardToken is BasicToken, ERC20 {
    mapping(address => mapping(address => uint256)) public allowed;
    uint256 public constant MAX_UINT256 = 2 ** 256 - 1;

    // @dev Transfer tokens from one address to another
    // @param _from address The address which you want to send tokens from
    // @param _to address The address which you want to transfer to
    // @param _value uint256 the amount of tokens to be transferred
    function transferFrom(address _from, address _to, uint256 _value) public onlyPayloadSize(3 * 32) returns (bool success) {
        require(_to != address(0));
        require(_value <= balances[_from]);
        uint256 _allowance = allowed[_from][msg.sender];
        require(_value <= _allowance);

        // @dev Treat 2^256-1 means unlimited allowance
        if (_allowance < MAX_UINT256)
            allowed[_from][msg.sender] = _allowance.sub(_value);
        balances[_from] = balances[_from].sub(_value);
        balances[_to] = balances[_to].add(_value);
        emit Transfer(_from, _to, _value);
        return true;
    }

    // @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
    // Beware that changing an allowance with this method brings the risk that someone may use both the old
    // and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
    // race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
    // https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
    // @param _spender The address which will spend the funds.
    // @param _value The amount of tokens to be spent.
    function approve(address _spender, uint256 _value) public onlyPayloadSize(2 * 32) returns (bool success) {
        allowed[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }

    // @dev approve should be called when allowed[_spender] == 0. To increment allowed value is better to use
    // @dev this function to avoid 2 calls (and wait until the first transaction is mined)
    // @param _spender The address which will spend the funds.
    // @param _addedValue The amount of tokens to be added to the allowance.
    function increaseApproval(address _spender, uint _addedValue) public returns (bool success) {
        allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(_addedValue);
        emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
        return true;
    }

    function decreaseApproval(address _spender, uint _subtractedValue) public returns (bool success) {
        uint oldValue = allowed[msg.sender][_spender];
        if (_subtractedValue >= oldValue) {
            allowed[msg.sender][_spender] = 0;
        } else {
            allowed[msg.sender][_spender] = oldValue.sub(_subtractedValue);
        }
        emit Approval(msg.sender, _spender, allowed[msg.sender][_spender]);
        return true;
    }

    // @dev Function to check the amount of tokens than an owner allowed to a spender.
    // @param _owner address The address which owns the funds.
    // @param _spender address The address which will spend the funds.
    // @return A uint256 specifying the amount of tokens still available for the spender.
    function allowance(address _owner, address _spender) public view returns (uint256 remaining) {
        return allowed[_owner][_spender];
    }
}
// @title Standard ERC20 token
// @dev Implementation of the basic standard token.
// @dev https://github.com/ethereum/EIPs/issues/20
// @dev Based oncode by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
function approve(address _spender, uint256 _value) public onlyPayloadSize(2 * 32) returns (bool success) {
    allowed[msg.sender][_spender] = _value;
    emit Approval(msg.sender, _spender, _value);
    return true;
}

// @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
// Beware that changing an allowance with this method brings the risk that someone may use both the old
// and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
// race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
// https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
// @param _spender The address which will spend the funds.
// @param _value The amount of tokens to be spent.