I want to substract BigInteger value from another BigInteger value, but I´ve got the following errors:

1.'BigInteger' is a type and cannot be used as an expression.
2.Syntax error in cast operator; two arguments separated by comma are required.
3.Value of type 'System.Numerics.BigInteger' cannot be converted to 'String'.

My code:

Code:
Option Strict On
Imports System.Numerics

Public Class Form3
    Dim number1 As BigInteger
    Dim number2 As BigInteger
    Dim result As BigInteger
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        number1 = CType(TextBox1.Text, BigInteger)
        number2 = CType(TextBox2.Text, BigInteger)
        result = CType(TextBox3.Text, BigInteger)
        result = CType(TextBox1.Text, BigInteger) - CType(Val(TextBox2.Text, BigInteger))
        TextBox3.Text = result
    End Sub
End Class
2-why two arguments?
3-why biginteger isn´t able to handle numbers directly?

Thanks.