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 A 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 to 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[msg.sender][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[msg.sender][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));
  }
}
function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
    _approve(msg.sender, spender, _allowed[msg.sender][spender].add(addedValue));
    return true;
  }
/**
 * @dev Increase the amount of tokens that an owner allowed to a spender.
 * approve should be called when _allowed[msg.sender][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.
 */