Excel PROS please : Is there any excel function that would help in finding different certain letters in 2 columns

For instance,

A      B
SSS   NNN
BBB   HHH
DDD   USS
GGG   PPP
DDD   SSS
DDD   USS
DDD   NNN

I would like to run a function like:
If any cell that contains any word with S as first letter in column A AND N as a first letter in the column B return "null" else return the result"*** ***"

if(LEFT A=D & B=U) return *** *** the result...
The result is 
DDD USS
DDD USS

Thanks in advanced 


