SunPower Inc., a solar energy company, plans to invest in research and development (R&D) to enhance its solar panel technology. The R&D process is anticipated to span three years, with an initial investment of $5 million. SunPower Inc. allocates $2 million in the first year, $1.5 million in the second year, and $1.5 million in the third year. After completing R&D, SunPower Inc. expects annual revenue of $10 million from the sale of advanced solar panels. Determine the payback period for this R&D investment.