contract ERC20 is IERC20 {
  using SafeMath for uint256;

  mapping (address => uint256) private _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 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) {
    _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) {
    _approve(msg.sender, spender, value);
    return true;
  }

  /**
   * @dev Transfer tokens from one address to another.
   * Note that while this function emits an Approval event, this is not required as per the specification,
   * and other compliant implementations may not emit the event.
   * @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)
  {
    _transfer(from, to, value);
    _approve(from, msg.sender, _allowed[from][msg.sender].sub(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
   * Emits an Approval event.
   * @param spender The address which will spend the funds.
   * @param addedValue The amount of tokens to increase the allowance by.
   */
  function increaseAllowance(
    address spender,
    uint256 addedValue
  )
    public
    returns (bool)
  {
    _approve(msg.sender, spender, _allowed[msg.sender][spender].add(addedValue));
    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
   * Emits an Approval event.
   * @param spender The address which will spend the funds.
   * @param subtractedValue The amount of tokens to decrease the allowance by.
   */
  function decreaseAllowance(
    address spender,
    uint256 subtractedValue
  )
    public
    returns (bool)
  {
    _approve(msg.sender, spender, _allowed[msg.sender][spender].sub(subtractedValue));
    return true;
  }

  /**
   * @dev Transfer token for a specified addresses
   * @param from The address to transfer from.
   * @param to The address to transfer to.
   * @param value The amount to be transferred.
   */
  function _transfer(address from, address to, uint256 value) internal {
    require(to != address(0));

    _balances[from] = _balances[from].sub(value);
    _balances[to] = _balances[to].add(value);
    emit Transfer(from, to, value);
  }

  /**
   * @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 value The amount that will be created.
   */
  function _mint(address account, uint256 value) internal {
    require(account != address(0));

    _totalSupply = _totalSupply.add(value);
    _balances[account] = _balances[account].add(value);
    emit Transfer(address(0), account, value);
  }

  /**
   * @dev Internal function that burns an amount of the token of a given
   * account.
   * @param account The account whose tokens will be burnt.
   * @param value The amount that will be burnt.
   */
  function _burn(address account, uint256 value) internal {
    require(account != address(0));

    _totalSupply = _totalSupply.sub(value);
    _balances[account] = _balances[account].sub(value);
    emit Transfer(account, address(0), value);
  }

  /**
   * @dev Approve an address to spend another addresses' tokens.
   * @param owner The address that owns the tokens.
   * @param spender The address that will spend the tokens.
   * @param value The number of tokens that can be spent.
   */
  function _approve(address owner, address spender, uint256 value) internal {
    require(spender != address(0));
    require(owner != address(0));

    _allowed[owner][spender] = value;
    emit Approval(owner, spender, value);
  }

  /**
   * @dev Internal function that burns an amount of the token of a given
   * account, deducting from the sender's allowance for said account. Uses the
   * internal burn function.
   * Emits an Approval event (reflecting the reduced allowance).
   * @param account The account whose tokens will be burnt.
   * @param value The amount that will be burnt.
   */
  function _burnFrom(address account, uint256 value) internal {
    _burn(account, value);
    _approve(account, msg.sender, _allowed[account][msg.sender].sub(value));
  }
}
/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
 * Originally based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 *
 * This implementation emits additional Approval events, allowing applications to reconstruct the allowance status for
 * all accounts just by listening to said events. Note that this isn't required by the specification, and other
 * compliant implementations may not do it.
 */
function totalSupply() public view returns (uint256) {
    return _totalSupply;
  }
/**
 * @dev Total number of tokens in existence
 */