An investor can design a risky portfolio based on two stocks, A and B. Stock A has an expected return of 45% and a standard deviation of return of 9%. Stock B has an expected return of 15% and a standard deviation of return of 2%.The correlation coefficient between the returns of A and B is 0.0025. The risk-free rate of return is 2%. The standard deviation of return on the optimal risky portfolio is _________.