An investor can design a risky portfolio based on two stocks, A and B. Stock A has an expected return of 9% and a standard deviation of return of 7%. Stock B has an expected return of 5% and a standard deviation of return of 6%.The correlation coefficient between the returns of A and B is -0.5. The risk-free rate of return is 3.5%. The standard deviation of return on the optimal risky portfolio is _________. Note: Express your answers in strictly numerical terms. For example, if the answer is 5%, write 0.05