If the return on stock A in year 1 was 15 %, in year 2 was -10 %, in year 3 was 13 % and in year 4 was 18 %, what was the standard deviation of returns for stock A over this four year period? (Round your answer to 1 decimal place and record without a percent sign. If your final answer is negative, place a minus sign before the number with no space between the sign and the number).