Bill has been adding funds to his investment account each year for the past 3 years. He started with an initial investment of $1,000. After earning a 10% return the first year, he added $3,000 to his portfolio. In this year his investments lost 5%. Undeterred, Bill added $2,000 the next year and earned a 2% return. Last year, discouraged by the recent results, he only added $500 to his portfolio, but in this final year his investments earned 8%. What was Bill's dollar-weighted average return for his investments?