Every probability satisfies: 0 ≤ P(A) ≤ 1.
P(A) = 0 means the event never happens (impossible). P(A) = 1 means it always happens (certain). P(A) = 0.5 means it happens half the time.
If you calculate P(A) = 1.2 or P(A) = -0.3, you made a mistake. Check your counting.