quantum mechanics - $lambda=frac{2h}{p}$ instead of $lambda=frac{h}{p}$?

I am studying quantum physics and there is something I don't understand:

1. 
   
   

   I know that for any particle $E=hf$ (Einstein relation) and

   
   


2. 
   

   $v=\lambda f$ ($v$ is the speed of the particle).

   
   


3. 
   

   I also know that the kinetic energy is $E_k=\frac{mv^2}{2}$.

   
   

Solving those 3 equations for $\lambda$: $$h\frac{v}{\lambda}=\frac{mv^2}{2},$$ I finally find $$\lambda=\frac{2h}{mv}=\frac{2h}{p},$$ which is not consistent with the De Broglie relation $$\lambda=\frac{h}{p}.$$ Where am I wrong in my development?