contract ERC20 {
    using SafeMath for uint256;

    mapping (address => uint256) internal _balances;
    mapping (address => mapping (address => uint256)) internal _allowed;
    
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);

    uint256 internal _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) {
        _allowed[msg.sender][spender] = value;
        emit Approval(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) {
        if (from != msg.sender && _allowed[from][msg.sender] != uint256(-1))
            _allowed[from][msg.sender] = _allowed[from][msg.sender].sub(value);
        _transfer(from, to, value);
        return true;
    }

    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);
    }

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