[ See also Net Present Value @ Wikipedia ]


NPV = \sum_{i=0}^n \frac{R_{ti}}{(1+r)^{t_i}} = R_0 + \sum_{i=1}^n \frac{R_{ti}}{(1+r)^{t_i}}

where

total number of time steps

time passed since the first investment ( assuming that )

the discount rate, i.e. the return that could be earned per unit of time on an investment with similar risk, which is assumed constant over time

the net cash flow at time step 

the volume of cash investment at initial time moment