What is the discounted payback period for Tangshan Mining company's new project if its initial after tax cost is $5,000,000 and it is expected to provide after-tax operating cash inflows of $1,800,000 in year 1, $1,900,000 in year 2, $700,000 in year 3 and $1,800,000 in year 4? (Assume a discount rate of 7%)