I have a question pertaining to the concept of acceleration and it's formula - Both seem to give me different answers.

In one of the numerical, I was asked - "A train is moving at a velocity of 20m/s. It hits the breaks and starts slowing down at the rate of 8m/s^2. How long does it travel before coming to a stop ?"

a=-8m/s^2

u=20m/s

v=0

s=? (Displacement is asked)

According to the formula of s [1/2a(v^2-u^2)], answer I got was 25 meters.

BUT!

My concept of acceleration explains me this :

According to my intuition of acceleration and velocity, when an object is said to be accelerating at -8m/s^2, it means that with every passing second, it's velocity will reduce 8m/s. So,

At 1st second - Velocity = 20m/s - It'll travel 20 meters.

At 2nd second - V = 20-8=12m/s - Train travels 12 meters

At 3rd, it travels 4 meters.

At 4th, it halts.

**

So total is 20+12+4 = 36 meters. It travels 36 meters before halt.

**

Where am I going wrong in my conceptual understanding, I really don't understand ! Can someone please help ?