a=100 
b=10  

1)c=a-b  
2)d=c*2-k2; k2=(b*2)  
3)e=d*2-k3;k3=k2*2  
4)f=e*2-k4;k4=k3*2  
30) (...)
z=c+d+e+f+(...)
z=?

I need a for Statement that calculates this mathematical sequence.

Thanks