Black–Scholes provides a mathematical model of a financial market containing derivative instruments, and the resultant formula for the price of European-styled options. The model is expressed as the Black–Scholes equation, a partial differential equation describing the changing price of the option over time; it is derived assuming log-normal, geometric Brownian motion (see Brownian model of financial markets). The key financial insight behind the model is that one can perfectly hedge the option by buying and selling the underlying asset in just the right way and consequently "eliminate risk", absenting the risk adjustment from the pricing ( V {\displaystyle V} , the value, or price, of the option, grows at r {\displaystyle r} , the risk-free rate; see Black–Scholes equation § Financial interpretation).[1][2] This hedge, in turn, implies that there is only one right price—in an arbitrage-free sense—for the option.