[ See also Net Present Value @ Wikipedia ]

NPV = \sum_{t=0}^n \frac{R_t}{(1+r)^t} = R_0 + \sum_{t=1}^N \frac{R_t}{(1+r)^t}

where

 – time passed since the first investment,

 – the discount rate, i.e. the return that could be earned per unit of time on an investment with similar risk

 – total number of time steps

 – the net cash flow at time step t. 

 – the initial investment at time step t = 0.