If the probability of an event is P, the probability of that event NOT happening is equal to 1-P:
Example: if you roll a six-sided dye, what the probability of getting 4.
==> P(4) = 1/6 & the probability of NOT GETTING 4 is P(NOT 4) =5/6
I could have written 1- 1/6 & you'll notice that 1-1/6 =5/6