Hex to Decimal conversion in postgresql

Hi,
Can anyone tell me how to convert hex to decimal conversion only through sql (don't want to create function). I tried 2 things first one worked when I pass value physically but second one got error message

1)
select x'80512b5e'::bigint as f

Results are : -2131666964

Above one worked well

2)
create table hello(num int, name char(15));
insert into hello values(1,'80f157ec');

select name::bigint as DecimalVallue from hello

Getting below exception when i issue above sql command

Error: Bad int8 external representation "80f157ec"


Any thoughts how to do that ?

Thanks
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