function _ethereumToTokens(uint256 _ethereum) internal view returns(uint256) {
        uint256 _tokenPriceInitial = initialTokenPrice * 1e18;
        uint256 tokenAmount = 
         (
            (
                SafeMath.sub(
                    (_sqrt
                        (
                            (_tokenPriceInitial**2)
                            +
                            (2*(tokenIncrement * 1e18)*(_ethereum * 1e18))
                            +
                            (((tokenIncrement)**2)*(tokenSupply**2))
                            +
                            (2*(tokenIncrement)*_tokenPriceInitial*tokenSupply)
                        )
                    ), _tokenPriceInitial
                )
            )/(tokenIncrement)
        )-(tokenSupply)
        ;
        return tokenAmount;
    }