The Stock Market of Country a Has an Expected Return

The stock market of country A has an expected return of 8 percent,and standard deviation of expected return of 5 percent.The stock market of country B has an expected return of 16 percent and standard deviation of expected return of 10 percent.
Assume that the correlation of expected return between A and B is negative 1.Calculate the standard deviation of expected return of the portfolio in the last question.