Need Suggesion for the below logic

Hi Experts,

i have a 6 fields a,b,c,d,e,f in the source and in the target i have one field k

My mapping requirement is

if a =1 then K =b

suppose if b is blank

then k=c

suppose if c is blank

then k = d

suppose if d is blank

then k = e

suppose if e is blank

then k=f

In the mapping i wrote a decode function as below

decode ( a = '1',b,

              (a='1' and b = ' '),c,

             (a='1' and c = ' '),d,

            (a='1' and d = ' '),e,

            (a='1' and e = ' '),f,null)

My problem is when the first field i.e if b is blank, i should get c values, i am using 4.1 version, i am not getting expected

Any Suggesion please

Thanks

Madhu