Physics and g = 2s t^-2

[A-Man]

So, I was studying for my physics exam (2d kinematics) and that formula in the title is driving me crazeh. Where did that 2 come from? We all know gravity is acceleration, and acceleration is only "s t^-2" (without the 2). Why are we multiplying by 2 with gravity? is there anything special about the gravity acceleration? And why is it that when we wanna get the distance we use "gt^2/2" ?? again, where did this extra 2 in the denominator come from? (i do remember that this 2 didn't exist for gravity in the past lessons) So yeah, please help!!!