contract ERC20 is IERC20 {
  using SafeMath for uint256;

  mapping (address => uint256) private _balances;

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

  uint256 private _totalSupply;

  /**
   * @dev See `IERC20.totalSupply`.
   */
  function totalSupply() public view returns (uint256) {
    return _totalSupply;
  }

  /**
   * @dev See `IERC20.balanceOf`.
   */
  function balanceOf(address account) public view returns (uint256) {
    return _balances[account];
  }

  /**
   * @dev See `IERC20.transfer`.
   *
   * Requirements:
   *
   * - `recipient` cannot be the zero address.
   * - the caller must have a balance of at least `amount`.
   */
  function transfer(address recipient, uint256 amount) public returns (bool) {
    _transfer(msg.sender, recipient, amount);
    return true;
  }

  /**
   * @dev See `IERC20.allowance`.
   */
  function allowance(address owner, address spender) public view returns (uint256) {
    return _allowances[owner][spender];
  }

  /**
   * @dev See `IERC20.approve`.
   *
   * Requirements:
   *
   * - `spender` cannot be the zero address.
   */
  function approve(address spender, uint256 value) public returns (bool) {
    _approve(msg.sender, spender, value);
    return true;
  }

  /**
   * @dev See `IERC20.transferFrom`.
   *
   * Emits an `Approval` event indicating the updated allowance. This is not
   * required by the EIP. See the note at the beginning of `ERC20`;
   *
   * Requirements:
   * - `sender` and `recipient` cannot be the zero address.
   * - `sender` must have a balance of at least `value`.
   * - the caller must have allowance for `sender`'s tokens of at least
   * `amount`.
   */
  function transferFrom(address sender, address recipient, uint256 amount) public returns (bool) {
    _transfer(sender, recipient, amount);
    _approve(sender, msg.sender, _allowances[sender][msg.sender].sub(amount));
    return true;
  }

  /**
   * @dev Atomically increases the allowance granted to `spender` by the caller.
   *
   * This is an alternative to `approve` that can be used as a mitigation for
   * problems described in `IERC20.approve`.
   *
   * Emits an `Approval` event indicating the updated allowance.
   *
   * Requirements:
   *
   * - `spender` cannot be the zero address.
   */
  function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
    _approve(msg.sender, spender, _allowances[msg.sender][spender].add(addedValue));
    return true;
  }

  /**
   * @dev Atomically decreases the allowance granted to `spender` by the caller.
   *
   * This is an alternative to `approve` that can be used as a mitigation for
   * problems described in `IERC20.approve`.
   *
   * Emits an `Approval` event indicating the updated allowance.
   *
   * Requirements:
   *
   * - `spender` cannot be the zero address.
   * - `spender` must have allowance for the caller of at least
   * `subtractedValue`.
   */
  function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
    _approve(msg.sender, spender, _allowances[msg.sender][spender].sub(subtractedValue));
    return true;
  }

  /**
   * @dev Moves tokens `amount` from `sender` to `recipient`.
   *
   * This is internal function is equivalent to `transfer`, and can be used to
   * e.g. implement automatic token fees, slashing mechanisms, etc.
   *
   * Emits a `Transfer` event.
   *
   * Requirements:
   *
   * - `sender` cannot be the zero address.
   * - `recipient` cannot be the zero address.
   * - `sender` must have a balance of at least `amount`.
   */
  function _transfer(address sender, address recipient, uint256 amount) internal {
    require(sender != address(0), "ERC20: transfer from the zero address");
    require(recipient != address(0), "ERC20: transfer to the zero address");

    _balances[sender] = _balances[sender].sub(amount);
    _balances[recipient] = _balances[recipient].add(amount);
    emit Transfer(sender, recipient, amount);
  }

  /** @dev Creates `amount` tokens and assigns them to `account`, increasing
   * the total supply.
   *
   * Emits a `Transfer` event with `from` set to the zero address.
   *
   * Requirements
   *
   * - `to` cannot be the zero address.
   */
  function _mint(address account, uint256 amount) internal {
    require(account != address(0), "ERC20: mint to the zero address");

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

  /**
  * @dev Destoys `amount` tokens from `account`, reducing the
  * total supply.
  *
  * Emits a `Transfer` event with `to` set to the zero address.
  *
  * Requirements
  *
  * - `account` cannot be the zero address.
  * - `account` must have at least `amount` tokens.
  */
  function _burn(address account, uint256 value) internal {
    require(account != address(0), "ERC20: burn from the zero address");

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

  /**
   * @dev Sets `amount` as the allowance of `spender` over the `owner`s tokens.
   *
   * This is internal function is equivalent to `approve`, and can be used to
   * e.g. set automatic allowances for certain subsystems, etc.
   *
   * Emits an `Approval` event.
   *
   * Requirements:
   *
   * - `owner` cannot be the zero address.
   * - `spender` cannot be the zero address.
   */
  function _approve(address owner, address spender, uint256 value) internal {
    require(owner != address(0), "ERC20: approve from the zero address");
    require(spender != address(0), "ERC20: approve to the zero address");

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

  /**
   * @dev Destoys `amount` tokens from `account`.`amount` is then deducted
   * from the caller's allowance.
   *
   * See `_burn` and `_approve`.
   */
  function _burnFrom(address account, uint256 amount) internal {
    _burn(account, amount);
    _approve(account, msg.sender, _allowances[account][msg.sender].sub(amount));
  }
}
function approve(address spender, uint256 value) public returns (bool) {
    _approve(msg.sender, spender, value);
    return true;
  }
/**
 * @dev See `IERC20.approve`.
 *
 * Requirements:
 *
 * - `spender` cannot be the zero address.
 */