I am trying to understand a paper on options titled "OPTION PRICING: A SIMPLIFIED APPROACH" [1]. In it option price is calculated as the expected payoff from the possible states of stock prices by binomial distribution approach.<br>
I am stuck at one step.<br>
What does the following sentences exactly mean?

>Now, the latter bracketed expression is the complementary binomial distribution function $\Phi[a; n,p]$. The first bracketed expression can also be interpreted as a complementary binomial distribution function $\Phi[a; n, p’]$.

Thank you in advance.

**Bibliography** 

[1] John C. COX and Stephen A. ROSS and Mark RUBINSTEIN "[Option Pricing: A Simplified Approach](http://static.stevereads.com/papers_to_read/option_pricing_a_simplified_approach.pdf)", Journal of Financial Economics 7 (1979) 229-263. [DOI: 10.1016/0304-405X(79)90015-1](https://doi.org/10.1016/0304-405X(79)90015-1)