contract ERC20 is IERC20 {
    using SafeMath for uint256;

    mapping (address => uint256) internal balances_;

    mapping (address => mapping (address => uint256)) private allowed_;

    uint256 private totalSupply_;

    /**
    * @dev Total number of tokens in existence
    */
    function totalSupply() public view returns (uint256) {
        return totalSupply_;
    }

    /**
    * @dev Gets the balance of the specified address.
    * @param _owner The address to query the the balance of.
    * @return An uint256 representing the amount owned by the passed address.
    */
    function balanceOf(address _owner) public view returns (uint256) {
        return balances_[_owner];
    }

  /**
   * @dev Function to check the amount of tokens that 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)
    {
        return allowed_[_owner][_spender];
    }

    /**
    * @dev Transfer token for a specified address
    * @param _to The address to transfer to.
    * @param _value The amount to be transferred.
    */
    function transfer(address _to, uint256 _value) public returns (bool) {
        require(_value <= balances_[msg.sender],"Invalid value");
        require(_to != address(0),"Invalid address");

        balances_[msg.sender] = balances_[msg.sender].sub(_value);
        balances_[_to] = balances_[_to].add(_value);
        emit Transfer(msg.sender, _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 returns (bool) {
        allowed_[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }

    /**
    * @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
      returns (bool)
    {
        require(_value <= balances_[_from],"Value is more than balance");
        require(_value <= allowed_[_from][msg.sender],"Value is more than alloved");
        require(_to != address(0),"Invalid address");

        balances_[_from] = balances_[_from].sub(_value);
        balances_[_to] = balances_[_to].add(_value);
        allowed_[_from][msg.sender] = allowed_[_from][msg.sender].sub(_value);
        emit Transfer(_from, _to, _value);
        return true;
    }

  /**
   * @dev Increase the amount of tokens that an owner allowed to a spender.
   * approve should be called when allowed_[_spender] == 0. To increment
   * allowed value is better to use this function to avoid 2 calls (and wait until
   * the first transaction is mined)
   * From MonolithDAO Token.sol
   * @param _spender The address which will spend the funds.
   * @param _addedValue The amount of tokens to increase the allowance by.
   */
    function increaseApproval(
        address _spender,
        uint256 _addedValue
    )
      public
      returns (bool)
    {
        allowed_[msg.sender][_spender] = (allowed_[msg.sender][_spender].add(_addedValue));
        emit Approval(msg.sender, _spender, allowed_[msg.sender][_spender]);
        return true;
    }

    /**
    * @dev Decrease the amount of tokens that an owner allowed to a spender.
    * approve should be called when allowed_[_spender] == 0. To decrement
    * allowed value is better to use this function to avoid 2 calls (and wait until
    * the first transaction is mined)
    * From MonolithDAO Token.sol
    * @param _spender The address which will spend the funds.
    * @param _subtractedValue The amount of tokens to decrease the allowance by.
    */
    function decreaseApproval(
        address _spender,
        uint256 _subtractedValue
    )
      public
      returns (bool)
    {
        uint256 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 Internal function that mints an amount of the token and assigns it to
    * an account. This encapsulates the modification of balances such that the
    * proper events are emitted.
    * @param _account The account that will receive the created tokens.
    * @param _amount The amount that will be created.
    */
    function _mint(address _account, uint256 _amount) internal {
        require(_account != 0,"Invalid address");
        totalSupply_ = totalSupply_.add(_amount);
        balances_[_account] = balances_[_account].add(_amount);
        emit Transfer(address(0), _account, _amount);
    }

    /**
    * @dev Internal function that burns an amount of the token of a given
    * account.
    * @param _account The account whose tokens will be burnt.
    * @param _amount The amount that will be burnt.
    */
    function _burn(address _account, uint256 _amount) internal {
        require(_account != 0,"Invalid address");
        require(_amount <= balances_[_account],"Amount is more than balance");

        totalSupply_ = totalSupply_.sub(_amount);
        balances_[_account] = balances_[_account].sub(_amount);
        emit Transfer(_account, address(0), _amount);
    }

}
/**
 * @title Standard ERC20 token
 * @dev Implementation of the basic standard token.
 */
function decreaseApproval(
    address _spender,
    uint256 _subtractedValue
)
  public
  returns (bool)
{
    uint256 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 Decrease the amount of tokens that an owner allowed to a spender.
 * approve should be called when allowed_[_spender] == 0. To decrement
 * allowed value is better to use this function to avoid 2 calls (and wait until
 * the first transaction is mined)
 * From MonolithDAO Token.sol
 * @param _spender The address which will spend the funds.
 * @param _subtractedValue The amount of tokens to decrease the allowance by.
 */