Comparation aritmetic with a nvarchar type field

Question

Hi,

 

I want to compare numerics values with a varchar type field, so i have this table (Account_Master_Group2):

 

 

 

where fields (Inicio and Fin) are nvarchar type, so i run this query:

 

 

 

but i got records out of the range from 50000000 and 89999999:

 

 

 

My question is: How can i handle that nvarchar fields as a numeric fields? to avoid records out of the range.

 

Thanks WORLD