Type casting text to Numeric - Query Error

Hi there,

I have the following definitions,

1. CREATE TABLE Sampletemp ( Sampleid serial NOT NULL,  SampleText character varying(50), CONSTRAINT Sampletemp_id PRIMARY KEY (Sampleid));

2. Insert into Sampletemp (SampleText) values ('Mr. Raja'),('Mr. Alex'),('1000'),('2500'),('555');

3. select sum(SampleText) as SampleText from Sampletemp;

ERROR:  function sum(character varying) does not exist
LINE 3:   select sum(SampleText) as SampleText from Sampletemp;
                 ^
HINT:  No function matches the given name and argument types. You may need to add explicit type casts.

4. select sum(SampleText :: int) as SampleText from Sampletemp;
ERROR:  invalid input syntax for integer: "Mr. Raja"
I am getting error while selecting using some aggregation.  

I would like to sum up these values, if it has TEXT (example Name)that can be ZERO while querying. I need answer as 4055.

Can i have your suggestion/solutions please?

Regards,
Vikram A