Formula Cannot Reference a Range on other sheets

Hi lovely people,

I am trying to make the following forumula work:

=LOOKUP(2,1/(((COUNTIF($A$1:A1, Input!$A$2:$A$200)=0)*(COUNTIF(Input!$A$2:$A$200, Input!$A$2:$A$200)>0.5))), Input!$A$2:$A$200)

The parts in bold are references to a range in another sheet on the document called "Input". Whenever I type the "Input!" in front of the range, it immediately deactivates. This is incredibly frustrating.

I've also tried =LOOKUP(2,1/(((COUNTIF($A$1:A1, 'Input!'$A$2:$A$200)=0)*(COUNTIF('Input!'$A$2:$A$200, 'Input!'$A$2:$A$200)>0.5))), 'Input!'$A$2:$A$200) 

Any ideas on how to change it?

Cheers
JT