In Visual Basic.Net, in order to declare a variable it must start with either an underscore or a letter followed by letters, numbers, and underscores(or nothing else if it starts with a letter).

Something else, if you're declaring a number then you should set the data type as a numeric data type such as an Integer, Double, Decimal, Short, etc. rather than a String because a String basically represents Text. So change your code to something like this:
Code:
Dim _1 as Integer = 1
'Or
Dim one As Integer = 1
Edit - Here is a list of data types offered in VB.Net: https://msdn.microsoft.com/en-us/library/47zceaw7.aspx