Please consider a portfolio of the following two stocks:

Johnson & Johnson, average return $7\%$ p$.$a$.,$ volatility $16\%$ p$.$a$.$

Walgreen Company, average return $10\%$ p$.$a$.,$ volatility $20\%$ p$.$a$.$

The correlation between these two stocks is $22\%.$ Please calculate the volatility $($standard deviation$)$ in $\%$ p$.$a$.$ of a portfolio consisting of $50\%$ Johnson & Johnson and $50\%$ Walgreen.