Currently, Bitcoin is considered an investment target with stocks and bonds held at a ratio of 6:4, respectively. The correlation coefficient between stocks, bonds, and bitcoin yields is zero, the stock yield is higher than the bond yield, and the bond yield and bitcoin yield are the same. However, the standard deviation of Bitcoin returns is three times greater than that of bond returns. If the investment goal is to minimize the risk of the portfolio measured by the standard deviation while maintaining the expected return on the current portfolio, how much bitcoin should be bought (or sold at short selling)?