Ashley wants to form a risky portfolio by investing half of her money in stocks and half in bonds. She puts half in a stock mutual fund that has an expected return of 27% and a standard deviation of 32%; and the rest in a corporate bond mutual fund that has an expected return of 18.5% and a standard deviation of 21.5%. The stock and bond funds have a correlation of -0.25. What is the standard deviation of Ashley's portfolio?